Analyte sensor

ABSTRACT

The present invention relates generally to biointerface membranes utilized with implantable devices, such as devices for the detection of analyte concentrations in a biological sample. More particularly, the invention relates to novel biointerface membranes, to devices and implantable devices including these membranes, methods for forming the biointerface membranes on or around the implantable devices, and to methods for monitoring glucose levels in a biological fluid sample using an implantable analyte detection device.

INCORPORATION BY REFERENCE TO RELATED APPLICATIONS

Any and all priority claims identified in the Application Data Sheet, orany correction thereto, are hereby incorporated by reference under 37CFR 1.57. This application is a continuation of U.S. application Ser.No. 11/439,630, filed May 23, 2006, which is a continuation-in-part ofU.S. application Ser. No. 11/077,715, filed Mar. 10, 2005, now U.S. Pat.No. 7,497,827, which claims the benefit of U.S. Provisional ApplicationNo. 60/587,787, filed Jul. 13, 2004, U.S. Provisional Application No.60/587,800, filed Jul. 13, 2004, U.S. Provisional Application No.60/614,683, filed Sep. 30, 2004, and U.S. Provisional Application No.60/614,764 filed Sep. 30, 2004. U.S. application Ser. No. 11/439,630also claims the benefit of U.S. Provisional Application No. 60/683,923filed May 23, 2005. Each of the aforementioned applications isincorporated by reference herein in its entirety, and each is herebyexpressly made a part of this specification.

FIELD OF THE INVENTION

The present invention relates generally to biointerface membranesutilized with implantable devices, such as devices for the detection ofanalyte concentrations in a biological sample. More particularly, theinvention relates to novel biointerface membranes, to devices andimplantable devices including these membranes, methods for forming thebiointerface membranes on or around the implantable devices, and tomethods for monitoring glucose levels in a biological fluid sample usingan implantable analyte detection device.

BACKGROUND OF THE INVENTION

One of the most heavily investigated analyte sensing devices is theimplantable glucose device for detecting glucose levels in hosts withdiabetes. Despite the increasing number of individuals diagnosed withdiabetes and recent advances in the field of implantable glucosemonitoring devices, currently used devices are unable to provide datasafely and reliably for certain periods of time. See Moatti-Sirat etal., Diabetologia, 35:224-30 (1992). There are two commonly used typesof subcutaneously implantable glucose sensing devices. These typesinclude those that are implanted transcutaneously and those that arewholly implanted.

SUMMARY OF THE INVENTION

Accordingly, in a first aspect, an analyte sensing device is providedadapted for short term insertion into a host's soft tissue, comprising asensor having an architecture with at least one dimension less thanabout 1 mm, wherein the architecture is configured to create afluid-filled pocket surrounding at least a portion of the sensor invivo.

In an embodiment of the first aspect, the sensor is configured tomeasure a signal that is indicative of a concentration of the analytewithin the fluid-filled pocket.

In an embodiment of the first aspect, the device further compriseselectronics operatively coupled to the sensor and configured fordetecting a signal from the sensor, wherein the signal is indicative ofa concentration of an analyte within the host.

In an embodiment of the first aspect, the device further comprises ahousing configured for placement adjacent to a host's skin, wherein atleast a portion of the electronics are disposed in the housing.

In an embodiment of the first aspect, the sensor is a transcutaneoussensor.

In an embodiment of the first aspect, the device further comprises aspacer covering at least a portion of the sensor.

In an embodiment of the first aspect, the spacer covers a sensingmechanism of the sensor.

In an embodiment of the first aspect, the spacer comprises abiointerface.

In an embodiment of the first aspect, the spacer comprises a porousmembrane configured to allow a body fluid to fill pores to thereby forma fluid-filled pocket.

In an embodiment of the first aspect, the device further comprises abioactive agent incorporated into the sensor.

In an embodiment of the first aspect, the bioactive agent is selectedfrom the group consisting of an anti-barrier cell agent, ananti-inflammatory agent, an anti-infective agent, a necrosing agent, ananesthetic, an inflammatory agent, a growth factor, an angiogenicfactor, an adjuvant, an immunosuppressive agent, an antiplatelet agent,an anticoagulant, an ACE inhibitor, a cytotoxic agent, a vascularizationcompound, and an anti-sense molecule.

In a second aspect, an analyte sensor for measuring an analyte in a hostis provided, the sensor comprising a sensor configured fortranscutaneous insertion into a host's skin, wherein the sensor has anarchitecture with at least one dimension less than about 1 mm; and abiointerface covering at least a portion of the sensor.

In an embodiment of the second aspect, the biointerface is configured toallow fluid influx.

In an embodiment of the second aspect, the sensor is a transcutaneoussensor.

In an embodiment of the second aspect, the device further compriseselectronics operatively coupled to the sensor and configured fordetecting a signal from the sensor.

In an embodiment of the second aspect, the electronics are inductivelycoupled to the sensor.

In an embodiment of the second aspect, the device further comprises ahousing configured for placement adjacent to the host's skin.

In an embodiment of the second aspect, the device further comprises abioactive agent incorporated into the sensor.

In an embodiment of the second aspect, the bioactive agent is selectedfrom the group consisting of an anti-barrier cell agent, ananti-inflammatory agent, an anti-infective agent, a necrosing agent, ananesthetic, an inflammatory agent, a growth factor, an angiogenicfactor, an adjuvant, an immunosuppressive agent, an antiplatelet agent,an anticoagulant, an ACE inhibitor, a cytotoxic agent, a vascularizationcompound, and an anti-sense molecule.

In an embodiment of the second aspect, the biointerface is configured toprovide a space for a body fluid to reside around the sensor in vivo.

In an embodiment of the second aspect, the biointerface is a porousmembrane.

In an embodiment of the second aspect, the biointerface covers at leasta sensing mechanism of the sensor.

In a third aspect, a method of detecting an analyte in a host isprovided, comprising: a) inserting a sensor through a host's skin andinto the host, wherein the sensor is a component of an analyte sensingdevice configured for transcutaneous insertion into the host, whereinthe sensor has an architecture with at least one dimension less thanabout 1 mm, and wherein a biointerface covers at least a portion of thesensor, whereby fluid flows into the biointerface upon insertion of thesensor into the host; b) detecting from the sensor a signal indicativeof a presence or a concentration of the analyte in the host; and c)removing the sensor from the host.

In an embodiment of the third aspect, the method further comprisesrepeating steps a) through c) after about 3 days or within about 3 daysor less.

In an embodiment of the third aspect, the method further comprisesrepeating steps a) through c) after about 5 days or within about 5 daysor less.

In an embodiment of the third aspect, the method further comprisesrepeating steps a) through c) after about 7 days or within about 7 daysor less.

In an embodiment of the third aspect, the method further comprisesrepeating steps a) through c) after about 10 days or within about 10days or less.

In an embodiment of the third aspect, the method further comprisescoupling an electronics unit to the sensor.

In an embodiment of the third aspect, the sensor is a transcutaneoussensor.

In an embodiment of the third aspect, the method further comprisesinductively coupling an electronics unit to the sensor.

In a fourth aspect, a wholly implantable sensing device is provided, thedevice comprising a sensor configured for implantation into a host andconfigured to detect an analyte in the host, wherein the sensor has anarchitecture with at least one dimension less than about 1 mm; a porousbiointerface covering at least a portion of the sensor; and electronicsoperatively coupled to the sensor.

In an embodiment of the fourth aspect, the electronics are operativelyconnected to the sensor within a body of the sensor.

In an embodiment of the fourth aspect, the device further comprises atether configured for operatively connecting the sensor to theelectronics.

In an embodiment of the fourth aspect, the electronics are inductivelycoupled to the sensor.

In an embodiment of the fourth aspect, the device further comprises amechanical spacer around the sensor or a protective framework around thesensor.

In an embodiment of the fourth aspect, the electronics are configured todetect a signal from the sensor and wherein the signal is indicative ofat least one of a presence and a concentration of the analyte within thebiointerface.

In an embodiment of the fourth aspect, the biointerface is anelectrospun biointerface.

In an embodiment of the fourth aspect, the biointerface is formeddirectly on the sensor.

In an embodiment of the fourth aspect, the biointerface is preformed andsubsequently applied to the sensor.

In an embodiment of the fourth aspect, the biointerface comprises amaterial selected from the group consisting of silicone,polytetrafluoroethylene, expanded polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyolefin, polyester,polycarbonate, biostable polytetrafluoroethylene, polyurethanehomopolymer, polyurethane copolymer, polyurethane terpolymer,polypropylene, polyvinylchloride, polyvinylidene fluoride, polyvinylalcohol, polybutylene terephthalate, polymethylmethacrylate, polyetherether ketone, polyamide, polyurethane, cellulosic polymer, poly(ethyleneoxide), poly(propylene oxide), poly(propylene oxide) copolymer,polysulfone, polysulfone, block copolymers thereof, di-block copolymersthereof, tri-block copolymers thereof, alternating copolymers thereof,random copolymers thereof, graft copolymers thereof, mixtures thereof,and blends thereof.

In an embodiment of the fourth aspect, the biointerface is a fibrousbiointerface.

In an embodiment of the fourth aspect, the biointerface comprises fibersless than about 6 microns in all but the longest dimension.

In an embodiment of the fourth aspect, passageways in the biointerfaceare configured to allow passage therein of invasive cells and to notallow extensive ingrowth of vascular tissue therein.

In an embodiment of the fourth aspect, the biointerface is an amorphousbiointerface.

In an embodiment of the fourth aspect, a majority of pores of thebiointerface are greater than about 0.6 microns in at least onedimension.

In an embodiment of the fourth aspect, the porous biointerface isconfigured to support tissue ingrowth.

In an embodiment of the fourth aspect, the biointerface is configured toresist barrier cell layer formation.

In an embodiment of the fourth aspect, the porous biointerface comprisesinterconnected pores.

In an embodiment of the fourth aspect, the biointerface furthercomprises a bioactive agent.

In an embodiment of the fourth aspect, the bioactive agent is selectedfrom the group consisting of an anti-barrier cell agent, ananti-inflammatory agent, an anti-infective agent, a necrosing agent, ananesthetic, an inflammatory agent, a growth factor, an angiogenicfactor, an adjuvant, an immunosuppressive agent, an antiplatelet agent,an anticoagulant, an ACE inhibitor, a cytotoxic agent, a vascularizationcompound, and an anti-sense molecule.

In an embodiment of the fourth aspect, the sensor comprises a workingelectrode embedded within the biointerface.

In an embodiment of the fourth aspect, the sensor further comprises atleast one of a reference electrode embedded within the biointerface ordeposited on a surface of the biointerface and a counter electrodeembedded within the biointerface or deposited on a surface of thebiointerface.

In a fifth aspect, a wholly implantable device for measuring an analytein a host is provided, the device comprising a sensor configured forinsertion into the host, wherein the sensor has an architecture with atleast one dimension less than about 1 mm, a porous biointerfacecomprising a solid portion and a plurality of passageways extending fromopenings in an exterior surface of the biointerface into an interiorportion of the biointerface, and electronics operatively coupled to thesensor.

In an embodiment of the fifth aspect, the passageways areinterconnected.

In an embodiment of the fifth aspect, a cavity size of the passagewaysis greater than about 0.6 microns in at least one dimension.

In an embodiment of the fifth aspect, the biointerface is configured toresist barrier cell layer formation.

In an embodiment of the fifth aspect, the biointerface is configured tohave a depth of greater than one passageway in three dimensionssubstantially throughout the entirety of a matrix comprising thebiointerface.

In an embodiment of the fifth aspect, the solid portion comprisesfibers.

In an embodiment of the fifth aspect, the solid portion comprises fibersless than about 6 microns in all but the longest dimension.

In an embodiment of the fifth aspect, the passageways are configured toallow passage of invasive cells therein and to not allow extensiveingrowth of vascular tissue therein.

In a sixth aspect, a method of detecting an analyte in a host isprovided, comprising wholly implanting an analyte sensing device withina host, the device comprising a sensor for measuring the analyte in thehost, wherein the sensor has an architecture with at least one dimensionless than about 1 mm, a porous biointerface covering at least a portionof the sensor, and an electronics unit operatively coupled to thesensor; allowing tissue ingrowth within the biointerface; and detectingfrom the sensor a signal indicative of at least one of a presence and aconcentration of the analyte in the host.

In an embodiment of the sixth aspect, the sensor is tethered to theelectronics unit.

In an embodiment of the sixth aspect, the sensor is inductively coupledto the electronics unit.

In an embodiment of the sixth aspect, the signal is indicative of apresence of the analyte in the host or a concentration of analyte in thehost.

In an embodiment of the sixth aspect, the biointerface is configured toprevent formation of a barrier-cell layer.

In an embodiment of the sixth aspect, the method further comprisesremoving the sensor from the host after at least about 1 month.

In a seventh aspect, a method for fabricating an analyte sensorconfigured for insertion into a host's soft tissue is provided, themethod comprising forming a biointerface having a plurality ofpassageways and a solid portion on at least a sensing portion of asensor, wherein the sensor is configured to measure an analyte in thehost, and wherein the sensor has an architecture with at least onedimension less than about 1 mm.

In an embodiment of the seventh aspect, the step of forming abiointerface comprises a method selected from the group consisting ofelectrospinning, writing, lyophilizing, wrapping, weaving, and molding.

In an embodiment of the seventh aspect, the step of forming abiointerface comprises electrospinning the biointerface onto the sensor,writing the biointerface onto the sensor, lyophilizing the biointerfaceonto the sensor, wrapping the biointerface onto the sensor, weaving thebiointerface onto the sensor, and molding the biointerface onto thesensor.

In an embodiment of the seventh aspect, the step of forming abiointerface comprises forming the biointerface directly on the sensor.

In an embodiment of the seventh aspect, the step of forming abiointerface comprises pre-forming the biointerface and then applyingthe preformed biointerface to the sensor.

In an embodiment of the seventh aspect, the step of forming abiointerface comprises pre-forming the biointerface and inserting thesensor into the preformed biointerface.

In an embodiment of the seventh aspect, the step of forming abiointerface comprises forming a selectively removable porogen on thesensor, wherein the porogen comprises particles formed onto the sensorand solidified to form a solidified mass of continuously interconnectedparticles; filling the porogen with a material; substantiallysolidifying the material; and removing the mass of continuouslyinterconnected particles from contact with the sensor and solidifiedmaterial to thereby form a solid portion that defines a plurality ofpassageways of the biointerface.

In an embodiment of the seventh aspect, the biointerface comprises amaterial selected from the group consisting of silicone,polytetrafluoroethylene, expanded polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyolefin, polyester,polycarbonate, biostable polytetrafluoroethylene, polyurethanehomopolymer, polyurethane copolymer, polyurethane terpolymer,polypropylene, polyvinylchloride, polyvinylidene fluoride, polyvinylalcohol, polybutylene terephthalate, polymethylmethacrylate, polyetherether ketone, polyamide, polyurethane, cellulosic polymer, poly(ethyleneoxide), poly(propylene oxide), poly(propylene oxide) copolymer,polysulfone, polysulfone, block copolymers thereof, di-block copolymersthereof, tri-block copolymers thereof, alternating copolymers thereof,random copolymers thereof, graft copolymers thereof, mixtures thereof,and blends thereof.

In an embodiment of the seventh aspect, the step of forming abiointerface comprises forming an amorphous biointerface.

In an embodiment of the seventh aspect, the biointerface comprises poresof at least about 20 microns.

In an embodiment of the seventh aspect, the step of forming abiointerface comprises forming a fibrous biointerface.

In an embodiment of the seventh aspect, the biointerface comprisesfibers less than about 6 microns in all but the longest dimension.

In an embodiment of the seventh aspect, the method further comprises astep of incorporating a bioactive agent into the biointerface.

In an embodiment of the seventh aspect, the bioactive agent is selectedfrom the group consisting of an anti-barrier cell agent, ananti-inflammatory agent, an anti-infective agent, a necrosing agent, ananesthetic, an inflammatory agent, a growth factor, an angiogenicfactor, an adjuvant, an immunosuppressive agent, an antiplatelet agent,an anticoagulant, an ACE inhibitor, a cytotoxic agent, a vascularizationcompound, and an anti-sense molecule.

In an embodiment of the seventh aspect, the step of forming abiointerface comprises writing a biointerface onto the sensor using acomputer-aided machine.

In an eighth aspect, a method for fabricating an analyte sensorconfigured to be wholly implanted in a host's soft tissue is provided,the method comprising providing a sensor configured to measure ananalyte in the host, wherein the sensor has an architecture with atleast one dimension less than about 1 mm; and coating a biointerfaceonto the sensor, the biointerface comprising a plurality of cavities anda solid portion.

In an embodiment of the eighth aspect, the cavities are interconnected.

In an embodiment of the eighth aspect, the coating step comprises amethod selected from the group consisting of electrospinning, writing,lyophilizing, wrapping, weaving, and molding.

In an embodiment of the eighth aspect, the method further comprises astep of curing the biointerface.

In an embodiment of the eighth aspect, the coating step comprisesforming a selectively removable porogen onto the sensor, wherein theporogen comprises particles formed onto the sensor and solidified toform a solidified mass of continuously interconnected particles; fillingthe porogen with a material; substantially solidifying the material; andremoving the mass of continuously interconnected particles from contactwith the sensor and solidified material to thereby form a solid portionthat defines a plurality of passageways of the biointerface.

In an embodiment of the eighth aspect, the biointerface comprises amaterial selected from the group consisting of silicone,polytetrafluoroethylene, expanded polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyolefin, polyester,polycarbonate, biostable polytetrafluoroethylene, polyurethanehomopolymer, polyurethane copolymer, polyurethane terpolymer,polypropylene, polyvinylchloride, polyvinylidene fluoride, polyvinylalcohol, polybutylene terephthalate, polymethylmethacrylate, polyetherether ketone, polyamide, polyurethane, cellulosic polymer, poly(ethyleneoxide), poly(propylene oxide), poly(propylene oxide) copolymer,polysulfone, polysulfone, block copolymers thereof, di-block copolymersthereof, tri-block copolymers thereof, alternating copolymers thereof,random copolymers thereof, graft copolymers thereof, mixtures thereof,and blends thereof.

In an embodiment of the eighth aspect, the biointerface is an amorphousbiointerface.

In an embodiment of the eighth aspect, the amorphous biointerface ismolded onto the sensor.

In an embodiment of the eighth aspect, the coating step comprisesforming a fibrous biointerface.

In an embodiment of the eighth aspect, the biointerface comprises fibersless than about 6 microns in all but the longest dimension.

In an embodiment of the eighth aspect, the method further comprises astep of incorporating a bioactive agent into the biointerface.

In an embodiment of the eighth aspect, the bioactive agent is selectedfrom the group consisting of an anti-barrier cell agent, ananti-inflammatory agent, an anti-infective agent, a necrosing agent, ananesthetic, an inflammatory agent, a growth factor, an angiogenicfactor, an adjuvant, a wound factor, an immunosuppressive agent, anantiplatelet agent, an anticoagulant, an ACE inhibitor, a cytotoxicagent, a vascularization compound, and an anti-sense molecule.

In an embodiment of the eighth aspect, the coating step compriseswriting a biointerface onto the sensor using a computer-aided machine.

In an embodiment of the eighth aspect, the method further comprises astep of curing the biointerface.

In a ninth aspect, a method for making an analyte sensor configured forinsertion into a host's soft tissue is provided, the method comprisingproviding a sensor configured to measure an analyte in a host, whereinthe sensor has an architecture with at least one dimension less thanabout 1 mm; and directly writing a porous biointerface, wherein theporous biointerface is written based on a predefined pattern stored in acomputer system.

In an embodiment of the ninth aspect, the method further comprises astep of curing the biointerface during direct writing step or after thedirect writing step.

In an embodiment of the ninth aspect, the porous biointerface isdirectly written onto the sensor.

In an embodiment of the ninth aspect, the porous biointerface isdirectly written onto a substrate and then applied to the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an illustration of classical three-layered foreign bodyresponse to a conventional synthetic membrane implanted under the skin.

FIG. 1B is a side schematic view of adipose cell contact with aninserted transcutaneous sensor or an implanted sensor.

FIG. 1C is a side schematic view of a biointerface membrane preventingadipose cell contact with an inserted transcutaneous sensor or animplanted sensor.

FIG. 2A is an expanded view of an exemplary embodiment of a continuousanalyte sensor.

FIG. 2B is a cross-sectional view through the sensor of FIG. 2A on lineB-B.

FIG. 3A is a side schematic view of a transcutaneous analyte sensor inone embodiment.

FIG. 3B is a side schematic view of a transcutaneous analyte sensor inan alternative embodiment.

FIG. 3C is a side schematic view of a wholly implantable analyte sensorin one embodiment.

FIG. 3D is a side schematic view of a wholly implantable analyte sensorin an alternative embodiment.

FIG. 3E is a side schematic view of a wholly implantable analyte sensorin another alternative embodiment.

FIG. 3F is a side view of one embodiment of an implanted sensorinductively coupled to an electronics unit within a functionally usefuldistance on the host's skin.

FIG. 3G is a side view of one embodiment of an implanted sensorinductively coupled to an electronics unit implanted in the host'stissue at a functionally useful distance.

FIG. 4A is a cross-sectional schematic view of a membrane of a preferredembodiment that facilitates vascularization of the first domain withoutbarrier cell layer formation.

FIG. 4B is a cross-sectional schematic view of the membrane of FIG. 2Ashowing contractile forces caused by the fibrous tissue of the FBR.

FIG. 5 is a flow chart that illustrates the process of forming abiointerface-coated small structured sensor in one embodiment.

FIG. 6 is a flow chart that illustrates the process of forming abiointerface-coated sensor in an alternative embodiment.

FIG. 7 is a flow chart that illustrates the process of forming abiointerface-coated sensor in another alternative embodiment.

FIG. 8 is a flow chart that illustrates the process of forming abiointerface-wrapped sensor in one embodiment.

FIG. 9 is a flow chart that illustrates the process of forming a sensingbiointerface in one embodiment.

FIG. 10A is a scanning electron micrograph showing a cross-sectionalview of a cut porous silicone tube. The scale line equals 500 μm.

FIG. 10B is a scanning electron micrograph of a sugar mold formed on asensor, prior to silicone application. The scale line equals 100 μm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description and examples illustrate a preferred embodimentof the present invention in detail. Those of skill in the art willrecognize that there are numerous variations and modifications of thisinvention that are encompassed by its scope. Accordingly, thedescription of a preferred embodiment should not be deemed to limit thescope of the present invention.

Definitions

In order to facilitate an understanding of the preferred embodiment, anumber of terms are defined below.

The term “biointerface membrane” as used herein is a broad term, and isto be given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to a permeable membrane thatfunctions as an interface between host tissue and an implantable device.

The term “barrier cell layer” as used herein is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to a part of a foreign bodyresponse that forms a cohesive monolayer of cells (for example,macrophages and foreign body giant cells) that substantially block thetransport of molecules and other substances to the implantable device.

The term “cell processes” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to pseudopodia of a cell.

The term “cellular attachment” as used herein is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to adhesion of cells and/or cellprocesses to a material at the molecular level, and/or attachment ofcells and/or cell processes to microporous material surfaces ormacroporous material surfaces. One example of a material used in theprior art that encourages cellular attachment to its porous surfaces isthe BIOPORE™ cell culture support marketed by Millipore (Bedford,Mass.), and as described in Brauker et al., U.S. Pat. No. 5,741,330.

The term “solid portions” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to portions of a membrane'smaterial having a mechanical structure that demarcates cavities, voids,or other non-solid portions.

The term “co-continuous” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to a solid portion or cavitywherein an unbroken curved line in three dimensions can be drawn betweentwo sides of a membrane.

The term “biostable” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to materials that are relatively resistant todegradation by processes that are encountered in vivo.

The terms “bioresorbable” or “bioabsorbable” as used herein are broadterms, and are to be given their ordinary and customary meaning to aperson of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation tomaterials that can be absorbed, or lose substance, in a biologicalsystem.

The terms “nonbioresorbable” or “nonbioabsorbable” as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation tomaterials that are not substantially absorbed, or do not substantiallylose substance, in a biological system.

The term “analyte” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to a substance or chemical constituent in abiological fluid (for example, blood, interstitial fluid, cerebralspinal fluid, lymph fluid or urine) that can be analyzed. Analytes caninclude naturally occurring substances, artificial substances,metabolites, and/or reaction products. In some embodiments, the analytefor measurement by the sensing regions, devices, and methods is glucose.However, other analytes are contemplated as well, including but notlimited to acarboxyprothrombin; acylcarnitine; adenine phosphoribosyltransferase; adenosine deaminase; albumin; alpha-fetoprotein; amino acidprofiles (arginine (Krebs cycle), histidine/urocanic acid, homocysteine,phenylalanine/tyrosine, tryptophan); andrenostenedione; antipyrine;arabinitol enantiomers; arginase; benzoylecgonine (cocaine);biotinidase; biopterin; c-reactive protein; carnitine; carnosinase; CD4;ceruloplasmin; chenodeoxycholic acid; chloroquine; cholesterol;cholinesterase; conjugated 1-β hydroxy-cholic acid; cortisol; creatinekinase; creatine kinase MM isoenzyme; cyclosporin A; d-penicillamine;de-ethylchloroquine; dehydroepiandrosterone sulfate; DNA (acetylatorpolymorphism, alcohol dehydrogenase, alpha 1-antitrypsin, cysticfibrosis, Duchenne/Becker muscular dystrophy, glucose-6-phosphatedehydrogenase, hemoglobin A, hemoglobin S, hemoglobin C, hemoglobin D,hemoglobin E, hemoglobin F, D-Punjab, beta-thalassemia, hepatitis Bvirus, HCMV, HIV-1, HTLV-1, Leber hereditary optic neuropathy, MCAD,RNA, PKU, Plasmodium vivax, sexual differentiation, 21-deoxycortisol);desbutylhalofantrine; dihydropteridine reductase; diptheria/tetanusantitoxin; erythrocyte arginase; erythrocyte protoporphyrin; esterase D;fatty acids/acylglycines; free β-human chorionic gonadotropin; freeerythrocyte porphyrin; free thyroxine (FT4); free tri-iodothyronine(FT3); fumarylacetoacetase; galactose/gal-1-phosphate;galactose-1-phosphate uridyltransferase; gentamicin; glucose-6-phosphatedehydrogenase; glutathione; glutathione perioxidase; glycocholic acid;glycosylated hemoglobin; halofantrine; hemoglobin variants;hexosaminidase A; human erythrocyte carbonic anhydrase I;17-alpha-hydroxyprogesterone; hypoxanthine phosphoribosyl transferase;immunoreactive trypsin; lactate; lead; lipoproteins ((a), B/A-1, β);lysozyme; mefloquine; netilmicin; phenobarbitone; phenytoin;phytanic/pristanic acid; progesterone; prolactin; prolidase; purinenucleoside phosphorylase; quinine; reverse tri-iodothyronine (rT3);selenium; serum pancreatic lipase; sissomicin; somatomedin C; specificantibodies (adenovirus, anti-nuclear antibody, anti-zeta antibody,arbovirus, Aujeszky's disease virus, dengue virus, Dracunculusmedinensis, Echinococcus granulosus, Entamoeba histolytica, enterovirus,Giardia duodenalisa, Helicobacter pylori, hepatitis B virus, herpesvirus, HIV-1, IgE (atopic disease), influenza virus, Leishmaniadonovani, leptospira, measles/mumps/rubella, Mycobacterium leprae,Mycoplasma pneumoniae, Myoglobin, Onchocerca volvulus, parainfluenzavirus, Plasmodium falciparum, poliovirus, Pseudomonas aeruginosa,respiratory syncytial virus, rickettsia (scrub typhus), Schistosomamansoni, Toxoplasma gondii, Trepenoma pallidium, Trypanosomacruzi/rangeli, vesicular stomatis virus, Wuchereria bancrofti, yellowfever virus); specific antigens (hepatitis B virus, HIV-1);succinylacetone; sulfadoxine; theophylline; thyrotropin (TSH); thyroxine(T4); thyroxine-binding globulin; trace elements; transferrin;UDP-galactose-4-epimerase; urea; uroporphyrinogen I synthase; vitamin A;white blood cells; and zinc protoporphyrin. Salts, sugar, protein, fat,vitamins, and hormones naturally occurring in blood or interstitialfluids can also constitute analytes in certain embodiments. The analytecan be naturally present in the biological fluid, for example, ametabolic product, a hormone, an antigen, an antibody, and the like.Alternatively, the analyte can be introduced into the body, for example,a contrast agent for imaging, a radioisotope, a chemical agent, afluorocarbon-based synthetic blood, or a drug or pharmaceuticalcomposition, including but not limited to insulin; ethanol; cannabis(marijuana, tetrahydrocannabinol, hashish); inhalants (nitrous oxide,amyl nitrite, butyl nitrite, chlorohydrocarbons, hydrocarbons); cocaine(crack cocaine); stimulants (amphetamines, methamphetamines, Ritalin,Cylert, Preludin, Didrex, PreState, Voranil, Sandrex, Plegine);depressants (barbituates, methaqualone, tranquilizers such as Valium,Librium, Miltown, Serax, Equanil, Tranxene); hallucinogens(phencyclidine, lysergic acid, mescaline, peyote, psilocybin); narcotics(heroin, codeine, morphine, opium, meperidine, Percocet, Percodan,Tussionex, Fentanyl, Darvon, Talwin, Lomotil); designer drugs (analogsof fentanyl, meperidine, amphetamines, methamphetamines, andphencyclidine, for example, Ecstasy); anabolic steroids; and nicotine.The metabolic products of drugs and pharmaceutical compositions are alsocontemplated analytes. Analytes such as neurochemicals and otherchemicals generated within the body can also be analyzed, such as, forexample, ascorbic acid, uric acid, dopamine, noradrenaline,3-methoxytyramine (3MT), 3,4-dihydroxyphenylacetic acid (DOPAC),homovanillic acid (HVA), 5-hydroxytryptamine (5HT),5-hydroxyindoleacetic acid (FHIAA), and histamine.

The term “host” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to mammals, preferably humans.

The phrase “continuous analyte sensing” as used herein is a broad term,and is to be given its ordinary and customary meaning to a person ofordinary skill in the art (and is not to be limited to a special orcustomized meaning), and refers without limitation to the period inwhich monitoring of analyte concentration is continuously, continually,and/or intermittently (but regularly) performed, for example, from aboutevery 5 seconds or less to about 10 minutes or more, preferably fromabout 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, or 60 second to about1.25, 1.50, 1.75, 2.00, 2.25, 2.50, 2.75, 3.00, 3.25, 3.50, 3.75, 4.00,4.25, 4.50, 4.75, 5.00, 5.25, 5.50, 5.75, 6.00, 6.25, 6.50, 6.75, 7.00,7.25, 7.50, 7.75, 8.00, 8.25, 8.50, 8.75, 9.00, 9.25, 9.50 or 9.75minutes.

The terms “analyte measuring device,” “sensor,” “sensing region,” and“sensing mechanism” as used herein are broad terms, and are to be giventheir ordinary and customary meaning to a person of ordinary skill inthe art (and are not to be limited to a special or customized meaning),and refer without limitation to the area of an analyte-monitoring deviceresponsible for the detection of a particular analyte. For example, thesensing region can comprise a non-conductive body, a working electrode,a reference electrode, and a counter electrode (optional), forming anelectrochemically reactive surface at one location on the body and anelectronic connection at another location on the body, and a sensingmembrane affixed to the body and covering the electrochemically reactivesurface. During general operation of the device, a biological sample,for example, blood or interstitial fluid, or a component thereofcontacts, either directly or after passage through one or moremembranes, an enzyme, for example, glucose oxidase. The reaction of thebiological sample or component thereof results in the formation ofreaction products that permit a determination of the analyte level, forexample, glucose, in the biological sample. In some embodiments, thesensing membrane further comprises an enzyme domain, for example, anenzyme layer, and an electrolyte phase, for example, a free-flowingliquid phase comprising an electrolyte-containing fluid describedfurther below. The terms are broad enough to include the entire device,or only the sensing portion thereof (or something in between).

The term “electrochemically reactive surface” as used herein is a broadterm, and is to be given its ordinary and customary meaning to a personof ordinary skill in the art (and is not to be limited to a special orcustomized meaning), and refers without limitation to the surface of anelectrode where an electrochemical reaction takes place. In a workingelectrode, hydrogen peroxide produced by an enzyme-catalyzed reaction ofan analyte being detected reacts can create a measurable electroniccurrent. For example, in the detection of glucose, glucose oxidaseproduces H₂O₂ peroxide as a byproduct. The H₂O₂ reacts with the surfaceof the working electrode to produce two protons (2H⁺), two electrons(2e⁻) and one molecule of oxygen (O₂), which produces the electroniccurrent being detected. In a counter electrode, a reducible species, forexample, O₂ is reduced at the electrode surface so as to balance thecurrent generated by the working electrode.

The term “sensing membrane” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to a permeable or semi-permeablemembrane that can comprise one or more domains and that is constructedof materials having a thickness of a few microns or more, and that arepermeable to reactants and/or co-reactants employed in determining theanalyte of interest. As an example, a sensing membrane can comprise animmobilized glucose oxidase enzyme, which catalyzes an electrochemicalreaction with glucose and oxygen to permit measurement of aconcentration of glucose.

The term “proximal” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to a region near to a point of reference, suchas an origin or a point of attachment.

The term “distal” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to a region spaced relatively far from a pointof reference, such as an origin or a point of attachment.

The terms “operably connected” and “operably linked” as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation to one ormore components linked to another component(s) in a manner thatfacilitates transmission of signals between the components. For example,one or more electrodes can be used to detect an analyte in a sample andconvert that information into a signal; the signal can then betransmitted to an electronic circuit. In this example, the electrode is“operably linked” to the electronic circuit.

The term “adhere” and “attach” as used herein are broad terms, and areto be given their ordinary and customary meaning to a person of ordinaryskill in the art (and are not to be limited to a special or customizedmeaning), and refer without limitation to hold, bind, or stick, forexample, by gluing, bonding, grasping, interpenetrating, or fusing.

The term “bioactive agent” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to any substance that has aneffect on or elicits a response from living tissue.

The term “bioerodible” or “biodegradable” as used herein are broadterms, and are to be given their ordinary and customary meaning to aperson of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation tomaterials that are enzymatically degraded or chemically degraded in vivointo simpler components.

The terms “small diameter sensor,” “small structured sensor,” and“micro-sensor” as used herein are broad terms, and are to be given theirordinary and customary meaning to a person of ordinary skill in the art(and are not to be limited to a special or customized meaning), andrefer without limitation to sensing mechanisms that are less than about2 mm in at least one dimension, and more preferably less than about 1 mmin at least one dimension. In some embodiments, the sensing mechanism(sensor) is less than about 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6,0.5, 0.4, 0.3, 0.2, or 0.1 mm. In some embodiments, the sensingmechanism is a needle-type sensor, wherein the diameter is less thanabout 1 mm, see, for example, U.S. Pat. No. 6,613,379 to Ward et al. andco-pending U.S. patent application Ser. No. 11/077,715, filed on Mar.10, 2005 and entitled, “TRANSCUTANEOUS ANALYTE SENSOR,” both of whichare incorporated herein by reference in their entirety. In somealternative embodiments, the sensing mechanism includes electrodesdeposited on a planar substrate, wherein the thickness of theimplantable portion is less than about 1 mm, see, for example U.S. Pat.No. 6,175,752 to Say et al. and U.S. Pat. No. 5,779,665 to Mastrototaroet al., both of which are incorporated herein by reference in theirentirety.

The term “electrospinning” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to a process by which fibers aredrawn out from a viscous polymer solution or melt by applying anelectric field to a droplet of the solution (most often at a metallicneedle tip). The electric field draws this droplet into a structurecalled a Taylor cone. If the viscosity and surface tension of thesolution are appropriately tuned, varicose breakup (electrospray) isavoided and a stable jet is formed. A bending instability results in awhipping process which stretches and elongates this fiber until it has adiameter of micrometers (or nanometers).

The terms “interferants” and “interfering species” as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation to effectsand/or species that interfere with the measurement of an analyte ofinterest in a sensor to produce a signal that does not accuratelyrepresent the analyte measurement. In one example of an electrochemicalsensor, interfering species are compounds with an oxidation potentialthat overlaps with the analyte to be measured.

The term “drift” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to a progressive increase or decrease insignal over time that is unrelated to changes in host systemic analyteconcentrations, such as host postprandial glucose concentrations, forexample. While not wishing to be bound by theory, it is believed thatdrift may be the result of a local decrease in glucose transport to thesensor, due to cellular invasion, which surrounds the sensor and forms aFBC, for example. It is also believed that an insufficient amount ofinterstitial fluid is surrounding the sensor, which results in reducedoxygen and/or glucose transport to the sensor, for example. An increasein local interstitial fluid may slow or reduce drift and thus improvesensor performance.

The term “sensing region” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the region of a monitoringdevice responsible for the detection of a particular analyte. Thesensing region generally comprises a non-conductive body, a workingelectrode (anode), a reference electrode (optional), and/or a counterelectrode (cathode) passing through and secured within the body formingelectrochemically reactive surfaces on the body and an electronicconnective means at another location on the body, and a multi-domainmembrane affixed to the body and covering the electrochemically reactivesurface.

The term “domain” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to a region of the membrane system that can bea layer, a uniform or non-uniform gradient (for example, an anisotropicregion of a membrane), or a portion of a membrane.

The term “membrane system” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to a permeable or semi-permeablemembrane that can be comprised of two or more domains and is typicallyconstructed of materials of a few microns thickness or more, which ispermeable to oxygen and is optionally permeable to, e.g., glucose oranother analyte. In one example, the membrane system comprises animmobilized glucose oxidase enzyme, which enables a reaction to occurbetween glucose and oxygen whereby a concentration of glucose can bemeasured.

The terms “processor module” and “microprocessor” as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation to acomputer system, state machine, processor, or the like designed toperform arithmetic or logic operations using logic circuitry thatresponds to and processes the basic instructions that drive a computer.

Overview

Devices and probes that are transcutaneously inserted or implanted intosubcutaneous tissue conventionally elicit a foreign body response (FBR),which includes invasion of inflammatory cells that ultimately forms aforeign body capsule (FBC), as part of the body's response to theintroduction of a foreign material. Specifically, insertion orimplantation of a device, for example, a glucose sensing device, canresult in an acute inflammatory reaction resolving to chronicinflammation with concurrent building of fibrotic tissue, such as isdescribed in detail above. Eventually, over a period of two to threeweeks, a mature FBC, including primarily contractile fibrous tissueforms around the device. See Shanker and Greisler, Inflammation andBiomaterials in Greco R S, ed., “Implantation Biology: The Host Responseand Biomedical Devices” pp 68-80, CRC Press (1994). The FBC surroundingconventional implanted devices has been shown to hinder or block thetransport of analytes across the device-tissue interface. Thus,continuous extended life analyte transport (e.g., beyond the first fewdays) in vivo has been conventionally believed to be unreliable orimpossible.

FIG. 1A is a schematic drawing that illustrates a classical FBR to aconventional cell-impermeable synthetic membrane 10 implanted under theskin. There are three main layers of a FBR. The innermost FBR layer 12,adjacent to the device, is composed generally of macrophages and foreignbody giant cells 14 (herein referred to as the “barrier cell layer”).These cells form a monolayer of closely opposed cells over the entiresurface of a microscopically smooth membrane, a macroscopically smooth(but microscopically rough) membrane, or a microporous (i.e., averagepore size of less than about 1 μm) membrane. A membrane can be adhesiveor non-adhesive to cells; however, its relatively smooth surface causesthe downward tissue contracture 21 (discussed below) to translatedirectly to the cells at the device-tissue interface 26. Theintermediate FBR layer 16 (herein referred to as the “fibrous zone”),lying distal to the first layer with respect to the device, is a widezone (about 30 to 100 μm) composed primarily of fibroblasts 18, fibrousmatrixes, and contractile fibrous tissue 20. The organization of thefibrous zone, and particularly the contractile fibrous tissue 20,contributes to the formation of the monolayer of closely opposed cellsdue to the contractile forces 21 around the surface of the foreign body(for example, membrane 10). The outermost FBR layer 22 is looseconnective granular tissue containing new blood vessels 24 (hereinreferred to as the “vascular zone”). Over time, this FBR tissue becomesmuscular in nature and contracts around the foreign body so that theforeign body remains tightly encapsulated. Accordingly, the downwardforces 21 press against the tissue-device interface 26, and without anycounteracting forces, aid in the formation of a barrier cell layer 14that blocks and/or refracts the transport of analytes 23 (for example,glucose) across the tissue-device interface 26.

A consistent feature of the innermost layers 12, 16 is that they aredevoid of blood vessels. This has led to widely supported speculationthat poor transport of molecules across the device-tissue interface 26is due to a lack of vascularization near the interface. See Scharp etal., World J. Surg., 8:221-229 (1984); and Colton et al., J. Biomech.Eng., 113:152-170 (1991). Previous efforts to overcome this problem havebeen aimed at increasing local vascularization at the device-tissueinterface, but have achieved only limited success.

Although local vascularization can aid in sustenance of local tissueover time, the presence of a barrier cell layer 14 prevents the passageof molecules that cannot diffuse through the layer. For example, whenapplied to an implantable glucose-measuring device, it is unlikely thatglucose would enter the cell via glucose transporters on one side of thecell and exit on the other side. Instead, it is likely that any glucosethat enters the cell is phosporylated and remains within the cell. Theonly cells known to facilitate transport of glucose from one side of thecell to another are endothelial cells. Consequently, little glucosereaches the implant's membrane through the barrier cell layer. The knownart purports to increase the local vascularization in order to increasesolute availability. See Brauker et al., U.S. Pat. No. 5,741,330.However, it has been observed by the inventors that once the monolayerof cells (barrier cell layer) is established adjacent to a membrane,increasing angiogenesis is not sufficient to increase transport ofmolecules such as glucose and oxygen across the device-tissue interface26. In fact, the barrier cell layer blocks and/or refracts the analytes23 from transport across the device-tissue interface 26.

Referring now to short-term sensors, or the short-term function oflong-term sensors, it is believed that certain aspects of the FBR in thefirst few days may play a role in noise. It has been observed that somesensors function more poorly during the first few hours after insertionthan they do later. This is exemplified by noise and/or a suppression ofthe signal during the first few hours (e.g., about 2 to about 24 hours)after insertion. These anomalies often resolve spontaneously after whichthe sensors become less noisy, have improved sensitivity, and are moreaccurate than during the early period. It has been observed that sometranscutaneous sensors and wholly implantable sensors are subject tonoise for a period of time after application to the host (i.e., insertedtranscutaneously or wholly implanted below the skin). “Noise,” as usedherein, is a broad term and is used in its ordinary sense, including,without limitation, a signal detected by the sensor that is unrelated toanalyte concentration and can result in reduced sensor performance. Onetype of noise has been observed during the few hours (e.g., about 2 toabout 24 hours) after sensor insertion. After the first 24 hours, thenoise often disappears, but in some hosts, the noise may last for aboutthree to four days

When a sensor is first inserted or implanted into the subcutaneoustissue, it comes into contact with a wide variety of possible tissueconformations. Subcutaneous tissue in different hosts may be relativelyfat free in cases of very athletic people, or may be mostly composed offat in the majority of people. Fat comes in a wide array of texturesfrom very white, puffy fat to very dense, fibrous fat. Some fat is veryyellow and dense looking; some is very clear, puffy, and white looking,while in other cases it is more red or brown. The fat may be severalinches thick or only 1 cm thick. It may be very vascular or relativelynonvascular. Many hosts with diabetes have some subcutaneous scar tissuedue to years of insulin pump use or insulin injection. At times, duringinsertion, sensors may come to rest in such a scarred area. Thesubcutaneous tissue may even vary greatly from one location to anotherin the abdomen of a given host. Moreover, by chance, the sensor may cometo rest near a more densely vascularized area or in a less vascularizedarea of a given host. While not wishing to be bound by theory, it isbelieved that creating a space between the sensor surface and thesurrounding cells, including formation of a fluid pocket surrounding thesensor, may enhance sensor performance.

FIG. 1B is a side schematic view of adipose cell contact with aninserted transcutaneous sensor or an implanted sensor. In this case, thesensor is firmly inserted into a small space with adipose cells pressingup against the surface. Close association of the adipose cells with thesensor can also occur, for example wherein the surface of the sensor ishydrophobic. For example, the adipose cells 200 may physically block thesurface of the sensor.

Typically adipose cells can be about 120 microns in diameter and aretypically fed by tiny capillaries 205. When the sensor is pressedagainst the fat tissue, very few capillaries may actually come near thesurface of the sensor. This may be analogous to covering the surface ofthe sensor with an impermeable material such as cellophane, for example.Even if there were a few small holes in the cellophane, the sensor'sfunction would likely be compromised. Additionally, the surroundingtissue has a low metabolic rate and therefore does not require highamounts of glucose and oxygen. While not wishing to be bound by theory,it is believed that, during this early period, the sensor's signal canbe noisy and the signal can be suppressed due to close association ofthe sensor surface with the adipose cells and decreased availability ofoxygen and glucose both for physical-mechanical reasons andphysiological reasons.

Referring now to long-term function of a sensor, after a few days to twoor more weeks of implantation, these devices typically lose theirfunction. In some applications, cellular attack or migration of cells tothe sensor can cause reduced sensitivity and/or function of the device,particularly after the first day of implantation. See also, for example,U.S. Pat. No. 5,791,344 and Gross et al. and “Performance Evaluation ofthe MiniMed Continuous Monitoring System During Host home Use,” DiabetesTechnology and Therapeutics, (2000) 2(1):49-56, which have reported aglucose oxidase-based device, approved for use in humans by the Food andDrug Administration, that functions well for several days followingimplantation but loses function quickly after the several days (e.g., afew days up to about 14 days).

It is believed that this lack of device function is most likely due tocells, such as polymorphonuclear cells and monocytes that migrate to thesensor site during the first few days after implantation. These cellsconsume local glucose and oxygen. If there is an overabundance of suchcells, they can deplete glucose and/or oxygen before it is able to reachthe device enzyme layer, thereby reducing the sensitivity of the deviceor rendering it non-functional. Further inhibition of device functioncan be due to inflammatory cells, for example, macrophages, thatassociate, for example, align at the interface, with the implantabledevice, and physically block the transport of glucose into the device,for example, by formation of a barrier cell layer. Additionally, theseinflammatory cells can biodegrade many artificial biomaterials (some ofwhich were, until recently, considered non-biodegradable). Whenactivated by a foreign body, tissue macrophages degranulate, releasinghypochlorite (bleach) and other oxidative species. Hypochlorite andother oxidative species are known to break down a variety of polymers.

Analyte sensors for in vivo use over various lengths of time have beendeveloped. For example, sensors to be used for a short period of time,such as about 1 to about 14 days, have been produced. Herein, thissensor will be referred to as a short-term sensor (STS). STS may be atranscutaneous device, in that a portion of the device may be insertedthrough the host's skin and into the underlying soft tissue while aportion of the device remains on the surface of the host's skin. In oneaspect, in order to overcome the problems associated with noise or othersensor function in the short-term (e.g., short term sensors or shortterm function of long term sensors), preferred embodiments employmaterials that promote formation of a fluid pocket around the sensor,for example architectures such as porous biointerface membrane ormatrices that create a space between the sensor and the surroundingtissue.

In some embodiments, a short-term sensor is provided with a spaceradapted to provide a fluid pocket between the sensor and the host'stissue. It is believed that this spacer, for example a biointerfacematerial, matrix, structure, and the like as described in more detailelsewhere herein, provides for oxygen and/or glucose transport to thesensor.

FIG. 1C is a side schematic view of a biointerface membrane as thespacer preventing adipose cell contact with an inserted transcutaneoussensor or an implanted sensor in one exemplary embodiment. In thisillustration, a porous biointerface membrane 34 surrounds the sensor 58,covering the sensing mechanism 34 and is configured to fill with fluidin vivo, thereby creating a fluid pocket surrounding the sensor.Accordingly, the adipose cells surrounding the sensor are held adistance away (such as the thickness of the porous biointerfacemembrane, for example) from the sensor surface. Accordingly, as theporous biointerface membrane fills with fluid (i.e., creates fluidpocket), oxygen and glucose are transported to the sensing mechanism inquantities sufficient to maintain accurate sensor function.

Accordingly, a short term sensor (or short term function of a long termsensor) including a biointerface, including but not limited to, forexample, porous biointerface materials, mesh cages, and the like, all ofwhich are described in more detail elsewhere herein, can be employed toimprove sensor function in the short term (e.g., first few hours todays). It is noted that porous biointerface membranes need notnecessarily include interconnected cavities for creating a fluid pocketin the short-term.

In some circumstances, for example in long-term sensors, it is believedthat that foreign body response is the dominant event surroundingextended implantation of an implanted device, and can be managed ormanipulated to support rather than hinder or block analyte transport. Inanother aspect, in order to extend the lifetime of the sensor, preferredembodiments employ materials that promote vascularized tissue ingrowth,for example within a porous biointerface membrane. For example tissuein-growth into a porous biointerface material surrounding a long-termsensor may promote sensor function over extended periods of time (e.g.,weeks, months, or years). It has been observed that in-growth andformation of a tissue bed can take up to 3 weeks. Tissue ingrowth andtissue bed formation is believed to be part of the foreign bodyresponse. As will be discussed herein, the foreign body response can bymanipulated by the use of porous biointerface materials that surroundthe sensor and promote ingrowth of tissue and microvasculature overtime. Long term use sensors (LTS), for use over a period of weeks,months or even years, have also been produced. LTS may be whollyimplantable, and placed within the host's soft tissue below the skin,for example.

Accordingly, a long term sensor including a biointerface, including butnot limited to, for example, porous biointerface materials including asolid portion and interconnected cavities, all of which are described inmore detail elsewhere herein, can be employed to improve sensor functionin the long term (e.g., after tissue ingrowth).

Sensing Mechanism

In general, the analyte sensors of the preferred embodiments include asensing mechanism 34 with a small structure (e.g., small structured-,micro- or small diameter sensor), for example, a needle-type sensor, inat least a portion thereof. As used herein a “small structure”preferably refers to an architecture with at least one dimension lessthan about 1 mm. The small structured sensing mechanism can bewire-based, substrate based, or any other architecture. In somealternative embodiments, the term “small structure” can also refer toslightly larger structures, such as those having their smallestdimension being greater than about 1 mm, however, the architecture(e.g., mass or size) is designed to minimize the foreign body responsedue to size and/or mass. In the preferred embodiments, a biointerfacemembrane is formed onto the sensing mechanism 34 as described in moredetail below.

FIG. 2A is an expanded view of an exemplary embodiment of a continuousanalyte sensor 34, also referred to as a transcutaneous analyte sensor,or needle-type sensor, particularly illustrating the sensing mechanism36. Preferably, the sensing mechanism comprises a small structure asdefined herein and is adapted for insertion under the host's skin, andthe remaining body of the sensor (e.g., electronics, etc.) can reside exvivo. In the illustrated embodiment, the analyte sensor 34, includes twoelectrodes, i.e., a working electrode 38 and at least one additionalelectrode 30, which may function as a counter and/or referenceelectrode, hereinafter referred to as the reference electrode 30.

In some exemplary embodiments, each electrode is formed from a fine wirewith a diameter of from about 0.001 or less to about 0.010 inches ormore, for example, and is formed from, e.g., a plated insulator, aplated wire, or bulk electrically conductive material. Although theillustrated electrode configuration and associated text describe onepreferred method of forming a transcutaneous sensor, a variety of knowntranscutaneous sensor configurations can be employed with thetranscutaneous analyte sensor system of the preferred embodiments, suchas are described in U.S. Pat. No. 6,695,860 to Ward et al., U.S. Pat.No. 6,565,509 to Say et al., U.S. Pat. No. 6,248,067 to Causey III etal., and U.S. Pat. No. 6,514,718 to Heller et al.

In preferred embodiments, the working electrode comprises a wire formedfrom a conductive material, such as platinum, platinum-iridium,palladium, graphite, gold, carbon, conductive polymer, alloys, or thelike. Although the electrodes can by formed by a variety ofmanufacturing techniques (bulk metal processing, deposition of metalonto a substrate, or the like), it can be advantageous to form theelectrodes from plated wire (e.g., platinum on steel wire) or bulk metal(e.g., platinum wire). It is believed that electrodes formed from bulkmetal wire provide superior performance (e.g., in contrast to depositedelectrodes), including increased stability of assay, simplifiedmanufacturability, resistance to contamination (e.g., which can beintroduced in deposition processes), and improved surface reaction(e.g., due to purity of material) without peeling or delamination.

The working electrode 38 is configured to measure the concentration ofan analyte. In an enzymatic electrochemical sensor for detectingglucose, for example, the working electrode measures the hydrogenperoxide produced by an enzyme catalyzed reaction of the analyte beingdetected and creates a measurable electronic current. For example, inthe detection of glucose wherein glucose oxidase produces hydrogenperoxide as a byproduct, hydrogen peroxide reacts with the surface ofthe working electrode producing two protons (2H⁺), two electrons (2e⁻)and one molecule of oxygen (O₂), which produces the electronic currentbeing detected.

The working electrode 38 is covered with an insulating material, forexample, a non-conductive polymer. Dip-coating, spray-coating,vapor-deposition, or other coating or deposition techniques can be usedto deposit the insulating material on the working electrode. In oneembodiment, the insulating material comprises parylene, which can be anadvantageous polymer coating for its strength, lubricity, and electricalinsulation properties. Generally, parylene is produced by vapordeposition and polymerization of para-xylylene (or its substitutedderivatives). However, any suitable insulating material can be used, forexample, fluorinated polymers, polyethyleneterephthalate, polyurethane,polyimide, other nonconducting polymers, or the like. Glass or ceramicmaterials can also be employed. Other materials suitable for use includesurface energy modified coating systems such as are marketed under thetrade names AMC18, AMC148, AMC141, and AMC321 by Advanced MaterialsComponents Express of Bellafonte, Pa. In some alternative embodiments,however, the working electrode may not require a coating of insulator.

Preferably, the reference electrode 30, which may function as areference electrode alone, or as a dual reference and counter electrode,is formed from silver, silver/silver chloride, or the like. Preferably,the electrodes are juxtapositioned and/or twisted with or around eachother; however other configurations are also possible. In one example,the reference electrode 30 is helically wound around the workingelectrode 38 as illustrated in FIG. 1B. The assembly of wires may thenbe optionally coated together with an insulating material, similar tothat described above, in order to provide an insulating attachment(e.g., securing together of the working and reference electrodes).

In embodiments wherein an outer insulator is disposed, a portion of thecoated assembly structure can be stripped or otherwise removed, forexample, by hand, excimer lasing, chemical etching, laser ablation,grit-blasting (e.g., with sodium bicarbonate or other suitable grit), orthe like, to expose the electroactive surfaces. Alternatively, a portionof the electrode can be masked prior to depositing the insulator inorder to maintain an exposed electroactive surface area. In oneexemplary embodiment, grit blasting is implemented to expose theelectroactive surfaces, preferably utilizing a grit material that issufficiently hard to ablate the polymer material, while beingsufficiently soft so as to minimize or avoid damage to the underlyingmetal electrode (e.g., a platinum electrode). Although a variety of“grit” materials can be used (e.g., sand, talc, walnut shell, groundplastic, sea salt, and the like), in some preferred embodiments, sodiumbicarbonate is an advantageous grit-material because it is sufficientlyhard to ablate, e.g., a parylene coating without damaging, e.g., anunderlying platinum conductor. One additional advantage of sodiumbicarbonate blasting includes its polishing action on the metal as itstrips the polymer layer, thereby eliminating a cleaning step that mightotherwise be necessary.

In some embodiments, a radial window is formed through the insulatingmaterial to expose a circumferential electroactive surface of theworking electrode. Additionally, sections of electroactive surface ofthe reference electrode are exposed. For example, the sections ofelectroactive surface can be masked during deposition of an outerinsulating layer or etched after deposition of an outer insulatinglayer. In some applications, cellular attack or migration of cells tothe sensor can cause reduced sensitivity and/or function of the device,particularly after the first day of implantation. However, when theexposed electroactive surface is distributed circumferentially about thesensor (e.g., as in a radial window), the available surface area forreaction can be sufficiently distributed so as to minimize the effect oflocal cellular invasion of the sensor on the sensor signal.Alternatively, a tangential exposed electroactive window can be formed,for example, by stripping only one side of the coated assemblystructure. In other alternative embodiments, the window can be providedat the tip of the coated assembly structure such that the electroactivesurfaces are exposed at the tip of the sensor. Other methods andconfigurations for exposing electroactive surfaces can also be employed.

Preferably, the above-exemplified sensor has an overall diameter of notmore than about 0.020 inches (about 0.51 mm), more preferably not morethan about 0.018 inches (about 0.46 mm), and most preferably not morethan about 0.016 inches (0.41 mm). In some embodiments, the workingelectrode has a diameter of from about 0.001 inches or less to about0.010 inches or more, preferably from about 0.002 inches to about 0.008inches, and more preferably from about 0.004 inches to about 0.005inches. The length of the window can be from about 0.1 mm (about 0.004inches) or less to about 2 mm (about 0.078 inches) or more, andpreferably from about 0.5 mm (about 0.02 inches) to about 0.75 mm (0.03inches). In such embodiments, the exposed surface area of the workingelectrode is preferably from about 0.000013 in² (0.0000839 cm²) or lessto about 0.0025 in² (0.016129 cm²) or more (assuming a diameter of fromabout 0.001 inches to about 0.010 inches and a length of from about0.004 inches to about 0.078 inches). The preferred exposed surface areaof the working electrode is selected to produce an analyte signal with acurrent in the picoAmp range, such as is described in more detailelsewhere herein. However, a current in the picoAmp range can bedependent upon a variety of factors, for example the electroniccircuitry design (e.g., sample rate, current draw, A/D converter bitresolution, etc.), the membrane system (e.g., permeability of theanalyte through the membrane system), and the exposed surface area ofthe working electrode. Accordingly, the exposed electroactive workingelectrode surface area can be selected to have a value greater than orless than the above-described ranges taking into considerationalterations in the membrane system and/or electronic circuitry. Inpreferred embodiments of a glucose sensor, it can be advantageous tominimize the surface area of the working electrode while maximizing thediffusivity of glucose in order to optimize the signal-to-noise ratiowhile maintaining sensor performance in both high and low glucoseconcentration ranges.

In some alternative embodiments, the exposed surface area of the working(and/or other) electrode can be increased by altering the cross-sectionof the electrode itself. For example, in some embodiments thecross-section of the working electrode can be defined by a cross, star,cloverleaf, ribbed, dimpled, ridged, irregular, or other non-circularconfiguration; thus, for any predetermined length of electrode, aspecific increased surface area can be achieved (as compared to the areaachieved by a circular cross-section). Increasing the surface area ofthe working electrode can be advantageous in providing an increasedsignal responsive to the analyte concentration, which in turn can behelpful in improving the signal-to-noise ratio, for example.

In some alternative embodiments, additional electrodes can be includedwithin the assembly, for example, a three-electrode system (working,reference, and counter electrodes) and/or an additional workingelectrode (e.g., an electrode which can be used to generate oxygen,which is configured as a baseline subtracting electrode, or which isconfigured for measuring additional analytes). Co-pending U.S. patentapplication Ser. No. 11/007,635, filed Dec. 7, 2004 and entitled“SYSTEMS AND METHODS FOR IMPROVING ELECTROCHEMICAL ANALYTE SENSORS” andU.S. patent application Ser. No. 11/004,561, filed Dec. 3, 2004 andentitled “CALIBRATION TECHNIQUES FOR A CONTINUOUS ANALYTE SENSOR”describe some systems and methods for implementing and using additionalworking, counter, and/or reference electrodes. In one implementationwherein the sensor comprises two working electrodes, the two workingelectrodes are juxtapositioned (e.g., extend parallel to each other),around which the reference electrode is disposed (e.g., helicallywound). In some embodiments wherein two or more working electrodes areprovided, the working electrodes can be formed in a double-, triple-,quad-, etc. helix configuration along the length of the sensor (forexample, surrounding a reference electrode, insulated rod, or othersupport structure). The resulting electrode system can be configuredwith an appropriate membrane system, wherein the first working electrodeis configured to measure a first signal comprising glucose and baselineand the additional working electrode is configured to measure a baselinesignal consisting of baseline only (e.g., configured to be substantiallysimilar to the first working electrode without an enzyme disposedthereon),In this way, the baseline signal can be subtracted from thefirst signal to produce a glucose-only signal that is substantially notsubject to fluctuations in the baseline and/or interfering species onthe signal. Accordingly, the above-described dimensions can be alteredas desired. Although the preferred embodiments illustrate one electrodeconfiguration including one bulk metal wire helically wound aroundanother bulk metal wire, other electrode configurations are alsocontemplated. In an alternative embodiment, the working electrodecomprises a tube with a reference electrode disposed or coiled inside,including an insulator there between. Alternatively, the referenceelectrode comprises a tube with a working electrode disposed or coiledinside, including an insulator there between. In another alternativeembodiment, a polymer (e.g., insulating) rod is provided, wherein theelectrodes are deposited (e.g., electro-plated) thereon. In yet anotheralternative embodiment, a metallic (e.g., steel) rod is provided, coatedwith an insulating material, onto which the working and referenceelectrodes are deposited. In yet another alternative embodiment, one ormore working electrodes are helically wound around a referenceelectrode.

While the methods of preferred embodiments are especially well suitedfor use with small structured-, micro- or small diameter sensors, themethods can also be suitable for use with larger diameter sensors, e.g.,sensors of 1 mm to about 2 mm or more in diameter.

In some alternative embodiments, the sensing mechanism includeselectrodes deposited on a planar substrate, wherein the thickness of theimplantable portion is less than about 1 mm, see, for example U.S. Pat.No. 6,175,752 to Say et al. and U.S. Pat. No. 5,779,665 to Mastrototaroet al., both of which are incorporated herein by reference in theirentirety.

Sensing Membrane

Preferably, a sensing membrane 32 is disposed over the electroactivesurfaces of the sensor 34 and includes one or more domains or layers. Ingeneral, the sensing membrane functions to control the flux of abiological fluid there through and/or to protect sensitive regions ofthe sensor from contamination by the biological fluid, for example. Someconventional electrochemical enzyme-based analyte sensors generallyinclude a sensing membrane that controls the flux of the analyte beingmeasured, protects the electrodes from contamination of the biologicalfluid, and/or provides an enzyme that catalyzes the reaction of theanalyte with a co-factor, for example. See, e.g., co-pending U.S. patentapplication Ser. No. 10/838,912, filed May 3, 2004 entitled “IMPLANTABLEANALYTE SENSOR” and U.S. patent application Ser. No. 11/077,715, filedMar. 10, 2005 and entitled “TRANSCUTANEOUS ANALYTE SENSOR” which areincorporated herein by reference in their entirety.

The sensing membranes of the preferred embodiments can include anymembrane configuration suitable for use with any analyte sensor (such asdescribed in more detail above). In general, the sensing membranes ofthe preferred embodiments include one or more domains, all or some ofwhich can be adhered to or deposited on the analyte sensor as isappreciated by one skilled in the art. In one embodiment, the sensingmembrane generally provides one or more of the following functions: 1)protection of the exposed electrode surface from the biologicalenvironment, 2) diffusion resistance (limitation) of the analyte, 3) acatalyst for enabling an enzymatic reaction, 4) limitation or blockingof interfering species, and 5) hydrophilicity at the electrochemicallyreactive surfaces of the sensor interface, such as described in theabove-referenced co-pending U.S. Patent Applications.

Electrode Domain

In some embodiments, the membrane system comprises an optional electrodedomain. The electrode domain is provided to ensure that anelectrochemical reaction occurs between the electroactive surfaces ofthe working electrode and the reference electrode, and thus theelectrode domain is preferably situated more proximal to theelectroactive surfaces than the enzyme domain. Preferably, the electrodedomain includes a semipermeable coating that maintains a layer of waterat the electrochemically reactive surfaces of the sensor, for example, ahumectant in a binder material can be employed as an electrode domain;this allows for the full transport of ions in the aqueous environment.The electrode domain can also assist in stabilizing the operation of thesensor by overcoming electrode start-up and drifting problems caused byinadequate electrolyte. The material that forms the electrode domain canalso protect against pH-mediated damage that can result from theformation of a large pH gradient due to the electrochemical activity ofthe electrodes.

In one embodiment, the electrode domain includes a flexible,water-swellable, hydrogel film having a “dry film” thickness of fromabout 0.05 micron or less to about 20 microns or more, more preferablyfrom about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1,1.5, 2, 2.5, 3, or 3.5 to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,15, 16, 17, 18, 19, or 19.5 microns, and more preferably from about 2,2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns. “Dry film”thickness refers to the thickness of a cured film cast from a coatingformulation by standard coating techniques.

In certain embodiments, the electrode domain is formed of a curablemixture of a urethane polymer and a hydrophilic polymer. Particularlypreferred coatings are formed of a polyurethane polymer havingcarboxylate functional groups and non-ionic hydrophilic polyethersegments, wherein the polyurethane polymer is crosslinked with a watersoluble carbodiimide (e.g.,1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC))) in the presence ofpolyvinylpyrrolidone and cured at a moderate temperature of about 50° C.

Preferably, the electrode domain is deposited by spray or dip-coatingthe electroactive surfaces of the sensor. More preferably, the electrodedomain is formed by dip-coating the electroactive surfaces in anelectrode solution and curing the domain for a time of from about 15 toabout 30 minutes at a temperature of from about 40 to about 55° C. (andcan be accomplished under vacuum (e.g., 20 to 30 mmHg)). In embodimentswherein dip-coating is used to deposit the electrode domain, a preferredinsertion rate of from about 1 to about 3 inches per minute, with apreferred dwell time of from about 0.5 to about 2 minutes, and apreferred withdrawal rate of from about 0.25 to about 2 inches perminute provide a functional coating. However, values outside of thoseset forth above can be acceptable or even desirable in certainembodiments, for example, dependent upon viscosity and surface tensionas is appreciated by one skilled in the art. In one embodiment, theelectroactive surfaces of the electrode system are dip-coated one time(one layer) and cured at 50° C. under vacuum for 20 minutes.

Although an independent electrode domain is described herein, in someembodiments, sufficient hydrophilicity can be provided in theinterference domain and/or enzyme domain (the domain adjacent to theelectroactive surfaces) so as to provide for the full transport of ionsin the aqueous environment (e.g. without a distinct electrode domain).

Interference Domain

In some embodiments, an optional interference domain is provided, whichgenerally includes a polymer domain that restricts the flow of one ormore interferants. In some embodiments, the interference domainfunctions as a molecular sieve that allows analytes and other substancesthat are to be measured by the electrodes to pass through, whilepreventing passage of other substances, including interferants such asascorbate and urea (see U.S. Pat. No. 6,001,067 to Shults). Some knowninterferants for a glucose-oxidase based electrochemical sensor includeacetaminophen, ascorbic acid, bilirubin, cholesterol, creatinine,dopamine, ephedrine, ibuprofen, L-dopa, methyldopa, salicylate,tetracycline, tolazamide, tolbutamide, triglycerides, and uric acid.

Several polymer types that can be utilized as a base material for theinterference domain include polyurethanes, polymers having pendant ionicgroups, and polymers having controlled pore size, for example. In oneembodiment, the interference domain includes a thin, hydrophobicmembrane that is non-swellable and restricts diffusion of low molecularweight species. The interference domain is permeable to relatively lowmolecular weight substances, such as hydrogen peroxide, but restrictsthe passage of higher molecular weight substances, including glucose andascorbic acid. Other systems and methods for reducing or eliminatinginterference species that can be applied to the membrane system of thepreferred embodiments are described in co-pending U.S. patentapplication Ser. No. 10/896,312 filed Jul. 21, 2004 and entitled“ELECTRODE SYSTEMS FOR ELECTROCHEMICAL SENSORS,” Ser. No. 10/991,353,filed Nov. 16, 2004 and entitled, “AFFINITY DOMAIN FOR AN ANALYTESENSOR,” Ser. No. 11/007,635, filed Dec. 7, 2004 and entitled “SYSTEMSAND METHODS FOR IMPROVING ELECTROCHEMICAL ANALYTE SENSORS” and Ser. No.11/004,561, filed Dec. 3, 2004 and entitled, “CALIBRATION TECHNIQUES FORA CONTINUOUS ANALYTE SENSOR.” In some alternative embodiments, adistinct interference domain is not included.

In preferred embodiments, the interference domain is deposited onto theelectrode domain (or directly onto the electroactive surfaces when adistinct electrode domain is not included) for a domain thickness offrom about 0.05 micron or less to about 20 microns or more, morepreferably from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45,0.5, 1, 1.5, 2, 2.5, 3, or 3.5 to about 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably fromabout 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns. Thickermembranes can also be useful, but thinner membranes are generallypreferred because they have a lower impact on the rate of diffusion ofhydrogen peroxide from the enzyme membrane to the electrodes.Unfortunately, the thin thickness of the interference domainsconventionally used can introduce variability in the membrane systemprocessing. For example, if too much or too little interference domainis incorporated within a membrane system, the performance of themembrane can be adversely affected.

Enzyme Domain

In preferred embodiments, the membrane system further includes an enzymedomain disposed more distally from the electroactive surfaces than theinterference domain (or electrode domain when a distinct interference isnot included). In some embodiments, the enzyme domain is directlydeposited onto the electroactive surfaces (when neither an electrode orinterference domain is included). In the preferred embodiments, theenzyme domain provides an enzyme to catalyze the reaction of the analyteand its co-reactant, as described in more detail below. Preferably, theenzyme domain includes glucose oxidase; however other oxidases, forexample, galactose oxidase or uricase oxidase, can also be used.

For an enzyme-based electrochemical glucose sensor to perform well, thesensor's response is preferably limited by neither enzyme activity norco-reactant concentration. Because enzymes, including glucose oxidase,are subject to deactivation as a function of time even in ambientconditions, this behavior is compensated for in forming the enzymedomain. Preferably, the enzyme domain is constructed of aqueousdispersions of colloidal polyurethane polymers including the enzyme.However, in alternative embodiments the enzyme domain is constructedfrom an oxygen enhancing material, for example, silicone, orfluorocarbon, in order to provide a supply of excess oxygen duringtransient ischemia. Preferably, the enzyme is immobilized within thedomain. See U.S. patent application Ser. No. 10/896,639 filed on Jul.21, 2004 and entitled “Oxygen Enhancing Membrane Systems for ImplantableDevice.”

In preferred embodiments, the enzyme domain is deposited onto theinterference domain for a domain thickness of from about 0.05 micron orless to about 20 microns or more, more preferably from about 0.05, 0.1,0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 toabout 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5microns, and more preferably from about 2, 2.5 or 3 microns to about3.5, 4, 4.5, or 5 microns. However in some embodiments, the enzymedomain is deposited onto the electrode domain or directly onto theelectroactive surfaces. Preferably, the enzyme domain is deposited byspray or dip coating. More preferably, the enzyme domain is formed bydip-coating the electrode domain into an enzyme domain solution andcuring the domain for from about 15 to about 30 minutes at a temperatureof from about 40 to about 55° C. (and can be accomplished under vacuum(e.g., 20 to 30 mmHg)). In embodiments wherein dip-coating is used todeposit the enzyme domain at room temperature, a preferred insertionrate of from about 1 inch per minute to about 3 inches per minute, witha preferred dwell time of from about 0.5 minutes to about 2 minutes, anda preferred withdrawal rate of from about 0.25 inch per minute to about2 inches per minute provide a functional coating. However, valuesoutside of those set forth above can be acceptable or even desirable incertain embodiments, for example, dependent upon viscosity and surfacetension as is appreciated by one skilled in the art. In one embodiment,the enzyme domain is formed by dip coating two times (namely, formingtwo layers) in a coating solution and curing at 50° C. under vacuum for20 minutes. However, in some embodiments, the enzyme domain can beformed by dip-coating and/or spray-coating one or more layers at apredetermined concentration of the coating solution, insertion rate,dwell time, withdrawal rate, and/or desired thickness.

Resistance Domain

In preferred embodiments, the membrane system includes a resistancedomain disposed more distal from the electroactive surfaces than theenzyme domain. Although the following description is directed to aresistance domain for a glucose sensor, the resistance domain can bemodified for other analytes and co-reactants as well.

There exists a molar excess of glucose relative to the amount of oxygenin blood; that is, for every free oxygen molecule in extracellularfluid, there are typically more than 100 glucose molecules present (seeUpdike et al., Diabetes Care 5:207-21(1982)). However, an immobilizedenzyme-based glucose sensor employing oxygen as co-reactant ispreferably supplied with oxygen in non-rate-limiting excess in order forthe sensor to respond linearly to changes in glucose concentration,while not responding to changes in oxygen concentration. Specifically,when a glucose-monitoring reaction is oxygen limited, linearity is notachieved above minimal concentrations of glucose. Without asemipermeable membrane situated over the enzyme domain to control theflux of glucose and oxygen, a linear response to glucose levels can beobtained only for glucose concentrations of up to about 40 mg/dL.However, in a clinical setting, a linear response to glucose levels isdesirable up to at least about 400 mg/dL.

The resistance domain includes a semi permeable membrane that controlsthe flux of oxygen and glucose to the underlying enzyme domain,preferably rendering oxygen in a non-rate-limiting excess. As a result,the upper limit of linearity of glucose measurement is extended to amuch higher value than that which is achieved without the resistancedomain. In one embodiment, the resistance domain exhibits an oxygen toglucose permeability ratio of from about 50:1 or less to about 400:1 ormore, preferably about 200:1. As a result, one-dimensional reactantdiffusion is adequate to provide excess oxygen at all reasonable glucoseand oxygen concentrations found in the subcutaneous matrix (See Rhodeset al., Anal. Chem., 66:1520-1529 (1994)).

In alternative embodiments, a lower ratio of oxygen-to-glucose can besufficient to provide excess oxygen by using a high oxygen solubilitydomain (for example, a silicone or fluorocarbon-based material ordomain) to enhance the supply/transport of oxygen to the enzyme domain.If more oxygen is supplied to the enzyme, then more glucose can also besupplied to the enzyme without creating an oxygen rate-limiting excess.In alternative embodiments, the resistance domain is formed from asilicone composition, such as is described in co-pending U.S.application Ser. No. 10/695,636 filed Oct. 28, 2003 and entitled,“SILICONE COMPOSITION FOR BIOCOMPATIBLE MEMBRANE.”

In a preferred embodiment, the resistance domain includes a polyurethanemembrane with both hydrophilic and hydrophobic regions to control thediffusion of glucose and oxygen to an analyte sensor, the membrane beingfabricated easily and reproducibly from commercially availablematerials. A suitable hydrophobic polymer component is a polyurethane,or polyetherurethaneurea. Polyurethane is a polymer produced by thecondensation reaction of a diisocyanate and a difunctionalhydroxyl-containing material. A polyurethaneurea is a polymer producedby the condensation reaction of a diisocyanate and a difunctionalamine-containing material. Preferred diisocyanates include aliphaticdiisocyanates containing from about 4 to about 8 methylene units.Diisocyanates containing cycloaliphatic moieties can also be useful inthe preparation of the polymer and copolymer components of the membranesof preferred embodiments. The material that forms the basis of thehydrophobic matrix of the resistance domain can be any of those known inthe art as appropriate for use as membranes in sensor devices and ashaving sufficient permeability to allow relevant compounds to passthrough it, for example, to allow an oxygen molecule to pass through themembrane from the sample under examination in order to reach the activeenzyme or electrochemical electrodes. Examples of materials which can beused to make non-polyurethane type membranes include vinyl polymers,polyethers, polyesters, polyamides, inorganic polymers such aspolysiloxanes and polycarbosiloxanes, natural polymers such ascellulosic and protein based materials, and mixtures or combinationsthereof.

In a preferred embodiment, the hydrophilic polymer component ispolyethylene oxide. For example, one useful hydrophobic-hydrophiliccopolymer component is a polyurethane polymer that includes about 20%hydrophilic polyethylene oxide. The polyethylene oxide portions of thecopolymer are thermodynamically driven to separate from the hydrophobicportions of the copolymer and the hydrophobic polymer component. The 20%polyethylene oxide-based soft segment portion of the copolymer used toform the final blend affects the water pick-up and subsequent glucosepermeability of the membrane.

In preferred embodiments, the resistance domain is deposited onto theenzyme domain to yield a domain thickness of from about 0.05 micron orless to about 20 microns or more, more preferably from about 0.05, 0.1,0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 toabout 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5microns, and more preferably from about 2, 2.5 or 3 microns to about3.5, 4, 4.5, or 5 microns. Preferably, the resistance domain isdeposited onto the enzyme domain by spray coating or dip coating. Incertain embodiments, spray coating is the preferred depositiontechnique. The spraying process atomizes and mists the solution, andtherefore most or all of the solvent is evaporated prior to the coatingmaterial settling on the underlying domain, thereby minimizing contactof the solvent with the enzyme. One additional advantage ofspray-coating the resistance domain as described in the preferredembodiments includes formation of a membrane system that substantiallyblocks or resists ascorbate (a known electrochemical interferant inhydrogen peroxide-measuring glucose sensors). While not wishing to bebound by theory, it is believed that during the process of depositingthe resistance domain as described in the preferred embodiments, astructural morphology is formed, characterized in that ascorbate doesnot substantially permeate there through.

In preferred embodiments, the resistance domain is deposited on theenzyme domain by spray-coating a solution of from about 1 wt. % to about5 wt. % polymer and from about 95 wt. % to about 99 wt. % solvent. Inspraying a solution of resistance domain material, including a solvent,onto the enzyme domain, it is desirable to mitigate or substantiallyreduce any contact with enzyme of any solvent in the spray solution thatcan deactivate the underlying enzyme of the enzyme domain.Tetrahydrofuran (THF) is one solvent that minimally or negligiblyaffects the enzyme of the enzyme domain upon spraying. Other solventscan also be suitable for use, as is appreciated by one skilled in theart.

Although a variety of spraying or deposition techniques can be used,spraying the resistance domain material and rotating the sensor at leastone time by 180° can provide adequate coverage by the resistance domain.Spraying the resistance domain material and rotating the sensor at leasttwo times by 120 degrees provides even greater coverage (one layer of360° coverage), thereby ensuring resistivity to glucose, such as isdescribed in more detail above.

In preferred embodiments, the resistance domain is spray-coated andsubsequently cured for a time of from about 15 to about 90 minutes at atemperature of from about 40 to about 60° C. (and can be accomplishedunder vacuum (e.g., 20 to 30 mmHg)). A cure time of up to about 90minutes or more can be advantageous to ensure complete drying of theresistance domain. While not wishing to be bound by theory, it isbelieved that complete drying of the resistance domain aids instabilizing the sensitivity of the glucose sensor signal. It reducesdrifting of the signal sensitivity over time, and complete drying isbelieved to stabilize performance of the glucose sensor signal in loweroxygen environments.

In one embodiment, the resistance domain is formed by spray-coating atleast six layers (namely, rotating the sensor seventeen times by 120°for at least six layers of 360° coverage) and curing at 50° C. undervacuum for 60 minutes. However, the resistance domain can be formed bydip-coating or spray-coating any layer or plurality of layers, dependingupon the concentration of the solution, insertion rate, dwell time,withdrawal rate, and/or the desired thickness of the resulting film.

Advantageously, sensors with the membrane system of the preferredembodiments, including an electrode domain and/or interference domain,an enzyme domain, and a resistance domain, provide stable signalresponse to increasing glucose levels of from about 40 to about 400mg/dL, and sustained function (at least 90% signal strength) even at lowoxygen levels (for example, at about 0.6 mg/L O₂). While not wishing tobe bound by theory, it is believed that the resistance domain providessufficient resistivity, or the enzyme domain provides sufficient enzyme,such that oxygen limitations are seen at a much lower concentration ofoxygen as compared to prior art sensors.

In preferred embodiments, a sensor signal with a current in the picoAmprange is preferred, which is described in more detail elsewhere herein.However, the ability to produce a signal with a current in the picoAmprange can be dependent upon a combination of factors, including theelectronic circuitry design (e.g., A/D converter, bit resolution, andthe like), the membrane system (e.g., permeability of the analytethrough the resistance domain, enzyme concentration, and/or electrolyteavailability to the electrochemical reaction at the electrodes), and theexposed surface area of the working electrode. For example, theresistance domain can be designed to be more or less restrictive to theanalyte depending upon to the design of the electronic circuitry,membrane system, and/or exposed electroactive surface area of theworking electrode.

Accordingly, in preferred embodiments, the membrane system is designedwith a sensitivity of from about 1 pA/mg/dL to about 100 pA/mg/dL,preferably from about 5 pA/mg/dL to 25 pA/mg/dL, and more preferablyfrom about 4 to about 7 pA/mg/dL. While not wishing to be bound by anyparticular theory, it is believed that membrane systems designed with asensitivity in the preferred ranges permit measurement of the analytesignal in low analyte and/or low oxygen situations. Namely, conventionalanalyte sensors have shown reduced measurement accuracy in low analyteranges due to lower availability of the analyte to the sensor and/orhave shown increased signal noise in high analyte ranges due toinsufficient oxygen necessary to react with the amount of analyte beingmeasured. While not wishing to be bound by theory, it is believed thatthe membrane systems of the preferred embodiments, in combination withthe electronic circuitry design and exposed electrochemical reactivesurface area design, support measurement of the analyte in the picoAmprange, which enables an improved level of resolution and accuracy inboth low and high analyte ranges not seen in the prior art.

Although sensors of some embodiments described herein include anoptional interference domain in order to block or reduce one or moreinterferants, sensors with the membrane system of the preferredembodiments, including an electrode domain, an enzyme domain, and aresistance domain, have been shown to inhibit ascorbate without anadditional interference domain. Namely, the membrane system of thepreferred embodiments, including an electrode domain, an enzyme domain,and a resistance domain, has been shown to be substantiallynon-responsive to ascorbate in physiologically acceptable ranges. Whilenot wishing to be bound by theory, it is believed that the process ofdepositing the resistance domain by spray coating, as described herein,results in a structural morphology that is substantially resistanceresistant to ascorbate.

Interference-Free Membrane Systems

In general, it is believed that appropriate solvents and/or depositionmethods can be chosen for one or more of the domains of the membranesystem that form one or more transitional domains such that interferantsdo not substantially permeate there through. Thus, sensors can be builtwithout distinct or deposited interference domains, which arenon-responsive to interferants. While not wishing to be bound by theory,it is believed that a simplified multilayer membrane system, more robustmultilayer manufacturing process, and reduced variability caused by thethickness and associated oxygen and glucose sensitivity of the depositedmicron-thin interference domain can be provided. Additionally, theoptional polymer-based interference domain, which usually inhibitshydrogen peroxide diffusion, is eliminated, thereby enhancing the amountof hydrogen peroxide that passes through the membrane system.

Oxygen Conduit

As described above, certain sensors depend upon an enzyme within themembrane system through which the host's bodily fluid passes and inwhich the analyte (for example, glucose) within the bodily fluid reactsin the presence of a co-reactant (for example, oxygen) to generate aproduct. The product is then measured using electrochemical methods, andthus the output of an electrode system functions as a measure of theanalyte. For example, when the sensor is a glucose oxidase based glucosesensor, the species measured at the working electrode is H₂O₂. Anenzyme, glucose oxidase, catalyzes the conversion of oxygen and glucoseto hydrogen peroxide and gluconate according to the following reaction:

Glucose+O₂→Gluconate+H₂O₂

Because for each glucose molecule reacted there is a proportional changein the product, H₂O₂, one can monitor the change in H₂O₂ to determineglucose concentration. Oxidation of H₂O₂ by the working electrode isbalanced by reduction of ambient oxygen, enzyme generated H₂O₂ and otherreducible species at a counter electrode, for example. See Fraser, D.M., “An Introduction to In Vivo Biosensing: Progress and Problems.” In“Biosensors and the Body,” D. M. Fraser, ed., 1997, pp. 1-56 John Wileyand Sons, New York))

In vivo, glucose concentration is generally about one hundred times ormore that of the oxygen concentration. Consequently, oxygen is alimiting reactant in the electrochemical reaction, and when insufficientoxygen is provided to the sensor, the sensor is unable to accuratelymeasure glucose concentration. Thus, depressed sensor function orinaccuracy is believed to be a result of problems in availability ofoxygen to the enzyme and/or electroactive surface(s).

Accordingly, in an alternative embodiment, an oxygen conduit (forexample, a high oxygen solubility domain formed from silicone orfluorochemicals) is provided that extends from the ex vivo portion ofthe sensor to the in vivo portion of the sensor to increase oxygenavailability to the enzyme. The oxygen conduit can be formed as a partof the coating (insulating) material or can be a separate conduitassociated with the assembly of wires that forms the sensor.

FIG. 2B is a cross-sectional view through the sensor of FIG. 2A on lineB-B, showing an exposed electroactive surface of at least a workingelectrode 38 surrounded by a sensing membrane. In general, the sensingmembranes of the preferred embodiments include a plurality of domains orlayers, for example, an interference domain 44, an enzyme domain 46, anda resistance domain 48, and may include additional domains, such as anelectrode domain, a cell impermeable domain, and/or an oxygen domain(not shown), such as described in more detail in the above-citedco-pending U.S. Patent Applications. However, it is understood that asensing membrane modified for other sensors, for example, by includingfewer or additional domains is within the scope of the preferredembodiments. In some embodiments, one or more domains of the sensingmembranes are formed from materials such as silicone,polytetrafluoroethylene, polyethylene-co-tetrafluoroethylene,polyolefin, polyester, polycarbonate, biostable polytetrafluoroethylene,homopolymers, copolymers, terpolymers of polyurethanes, polypropylene(PP), polyvinylchloride (PVC), polyvinylidene fluoride (PVDF),polybutylene terephthalate (PBT), polymethylmethacrylate (PMMA),polyether ether ketone (PEEK), polyurethanes, cellulosic polymers,poly(ethylene oxide), poly(propylene oxide) and copolymers and blendsthereof, polysulfones and block copolymers thereof including, forexample, di-block, tri-block, alternating, random and graft copolymers.Co-pending U.S. patent application Ser. No. 10/838,912, which isincorporated herein by reference in its entirety, describes biointerfaceand sensing membrane configurations and materials that may be applied tothe preferred embodiments.

The sensing membrane can be deposited on the electroactive surfaces ofthe electrode material using known thin or thick film techniques (forexample, spraying, electro-depositing, dipping, or the like). It isnoted that the sensing membrane that surrounds the working electrodedoes not have to be the same structure as the sensing membrane thatsurrounds a reference electrode, etc. For example, the enzyme domaindeposited over the working electrode does not necessarily need to bedeposited over the reference and/or counter electrodes.

In the illustrated embodiment, the sensor is an enzyme-basedelectrochemical sensor, wherein the working electrode 38 measures thehydrogen peroxide produced by the enzyme catalyzed reaction of glucosebeing detected and creates a measurable electronic current (for example,detection of glucose utilizing glucose oxidase produces hydrogenperoxide as a by-product, H₂O₂ reacts with the surface of the workingelectrode producing two protons (2H⁺), two electrons (2e⁻) and onemolecule of oxygen (O₂) which produces the electronic current beingdetected), such as described in more detail above and as is appreciatedby one skilled in the art. Preferably, one or more potentiostat isemployed to monitor the electrochemical reaction at the electroactivesurface of the working electrode(s). The potentiostat applies a constantpotential to the working electrode and its associated referenceelectrode to determine the current produced at the working electrode.The current that is produced at the working electrode (and flows throughthe circuitry to the counter electrode) is substantially proportional tothe amount of H₂O₂ that diffuses to the working electrode. The outputsignal is typically a raw data stream that is used to provide a usefulvalue of the measured analyte concentration in a host to the host ordoctor, for example.

Some alternative analyte sensors that can benefit from the systems andmethods of the preferred embodiments include U.S. Pat. No. 5,711,861 toWard et al., U.S. Pat. No. 6,642,015 to Vachon et al., U.S. Pat. No.6,654,625 to Say et al., U.S. Pat. No. 6,565,509 to Say et al., U.S.Pat. No. 6,514,718 to Heller, U.S. Pat. No. 6,465,066 to Essenpreis etal., U.S. Pat. No. 6,214,185 to Offenbacher et al., U.S. Pat. No.5,310,469 to Cunningham et al., and U.S. Pat. No. 5,683,562 to Shafferet al., U.S. Pat. No. 6,579,690 to Bonnecaze et al., U.S. Pat. No.6,484,046 to Say et al., U.S. Pat. No. 6,512,939 to Colvin et al., U.S.Pat. No. 6,424,847 to Mastrototaro et al., U.S. Pat. No. 6,424,847 toMastrototaro et al., for example. All of the above patents areincorporated in their entirety herein by reference and are not inclusiveof all applicable analyte sensors; in general, it should be understoodthat the disclosed embodiments are applicable to a variety of analytesensor configurations.

Exemplary Sensor Configurations

FIG. 3A is a side schematic view of a transcutaneous analyte sensor 50in one embodiment. The sensor 50 includes a mounting unit 52 adapted formounting on the skin of a host, a small diameter sensor 34 (as definedherein) adapted for transdermal insertion through the skin of a host,and an electrical connection configured to provide secure electricalcontact between the sensor and the electronics preferably housed withinthe mounting unit 52. In general, the mounting unit 52 is designed tomaintain the integrity of the sensor in the host so as to reduce oreliminate translation of motion between the mounting unit, the host,and/or the sensor. See co-pending U.S. patent application Ser. No.11/077,715 filed on Mar. 10, 2005 and entitled, “TRANSCUTANEOUS ANALYTESENSOR,” which is incorporated herein by reference in its entirety.Preferably, a biointerface membrane is formed onto the sensing mechanism34 as described in more detail below.

FIG. 3B is a side schematic view of a transcutaneous analyte sensor 54in an alternative embodiment. The sensor 54 includes a mounting unit 52wherein the sensing mechanism 34 comprises a small structure as definedherein and is tethered to the mounting unit 52 via a cable 56(alternatively, a wireless connection can be utilized). The mountingunit is adapted for mounting on the skin of a host and is operablyconnected via a tether, or the like, to a small structured sensor 34adapted for transdermal insertion through the skin of a host andmeasurement of the analyte therein; see, for example, U.S. Pat. No.6,558,330 to Causey III et al., which is incorporated herein byreference in its entirety. In the preferred embodiments, a biointerfacemembrane is formed onto the sensing mechanism 34 as described in moredetail below.

The short-term sensor of the preferred embodiments may be inserted intoa variety of locations on the host's body, such as the abdomen, thethigh, the upper arm, and the neck or behind the ear. Although thepreferred embodiments illustrate insertion through the abdominal region,the systems and methods described herein are limited neither to theabdominal nor to the subcutaneous insertions. One skilled in the artappreciates that these systems and methods may be implemented and/ormodified for other insertion sites and may be dependent upon the type,configuration, and dimensions of the analyte sensor.

In one embodiment, an analyte-sensing device adapted for transcutaneousshort-term insertion into the host is provided. For example, the deviceincludes a sensor, for measuring the analyte in the host, a porous,biocompatible matrix covering at least a portion of the sensor, and anapplicator, for inserting the sensor through the host's skin. In someembodiments, the sensor has architecture with at least one dimensionless than about 1 mm. Examples of such a structure are shown in FIGS. 3Aand 3B, as described elsewhere herein. However, one skilled in the artwill recognize that alternative configurations are possible and may bedesirable, depending upon factors such as intended location ofinsertion, for example. The sensor is inserted through the host's skinand into the underlying tissue, such as soft tissue or fatty tissue.

After insertion, fluid moves into the spacer, e.g., a biocompatiblematrix or membrane, creating a fluid-filled pocket therein. This processmay occur immediately or may take place over a period of time, such asseveral minutes or hours post insertion. A signal from the sensor isthen detected, such as by the sensor electronics unit located in themounting unit on the surface of the host's skin. In general, the sensormay be used continuously for a short period of days, such as 1 to 14days. After use, the sensor is simply removed from the host's skin. Inpreferred embodiments, the host may repeat the insertion and detectionsteps as many times as desired. In some implementations, the sensor maybe removed after about 3 days, and then another sensor inserted, and soon. Similarly in other implementations, the sensor is removed afterabout 3, 5, 7, 10 or 14 days, followed by insertion of a new sensor, andso on.

Some examples of transcutaneous analyte sensors are described inco-pending U.S. patent application Ser. No. 11/360,250 and entitled“ANALYTE SENSOR,” which is incorporated herein by reference in itsentirety. In general, transcutaneous analyte sensors comprise the sensorand a mounting unit with electronics associated therewith.

In general, the mounting unit includes a base adapted for mounting onthe skin of a host, a sensor adapted for transdermal insertion throughthe skin of a host, and one or more contacts configured to providesecure electrical contact between the sensor and the sensor electronics.The mounting unit is designed to maintain the integrity of the sensor inthe host so as to reduce or eliminate translation of motion between themounting unit, the host, and/or the sensor. The base can be formed froma variety of hard or soft materials, and preferably comprises a lowprofile for minimizing protrusion of the device from the host duringuse. In some embodiments, the base is formed at least partially from aflexible material, which is believed to provide numerous advantages overconventional transcutaneous sensors, which, unfortunately, can sufferfrom motion-related artifacts associated with the host's movement whenthe host is using the device. For example, when a transcutaneous analytesensor is inserted into the host, various movements of the sensor (forexample, relative movement between the in vivo portion and the ex vivoportion, movement of the skin, and/or movement within the host (dermisor subcutaneous)) create stresses on the device and can produce noise inthe sensor signal. It is believed that even small movements of the skincan translate to discomfort and/or motion-related artifact, which can bereduced or obviated by a flexible or articulated base. Thus, byproviding flexibility and/or articulation of the device against thehost's skin, better conformity of the sensor system to the regular useand movements of the host can be achieved. Flexibility or articulationis believed to increase adhesion (with the use of an adhesive pad) ofthe mounting unit onto the skin, thereby decreasing motion-relatedartifact that can otherwise translate from the host's movements andreduced sensor performance.

In certain embodiments, the mounting unit is provided with an adhesivepad, preferably disposed on the mounting unit's back surface andpreferably including a releasable backing layer. Thus, removing thebacking layer and pressing the base portion of the mounting unit ontothe host's skin adheres the mounting unit to the host's skin.Additionally or alternatively, an adhesive pad can be placed over someor all of the sensor system after sensor insertion is complete to ensureadhesion, and optionally to ensure an airtight seal or watertight sealaround the wound exit-site (or sensor insertion site). Appropriateadhesive pads can be chosen and designed to stretch, elongate, conformto, and/or aerate the region (e.g., host's skin).

In preferred embodiments, the adhesive pad is formed from spun-laced,open- or closed-cell foam, and/or non-woven fibers, and includes anadhesive disposed thereon, however a variety of adhesive padsappropriate for adhesion to the host's skin can be used, as isappreciated by one skilled in the art of medical adhesive pads. In someembodiments, a double-sided adhesive pad is used to adhere the mountingunit to the host's skin. In other embodiments, the adhesive pad includesa foam layer, for example, a layer wherein the foam is disposed betweenthe adhesive pad's side edges and acts as a shock absorber.

In some embodiments, the surface area of the adhesive pad is greaterthan the surface area of the mounting unit's back surface.Alternatively, the adhesive pad can be sized with substantially the samesurface area as the back surface of the base portion. Preferably, theadhesive pad has a surface area on the side to be mounted on the host'sskin that is greater than about 1, 1.25, 1.5, 1.75, 2, 2.25, or 2.5,times the surface area of the back surface of the mounting unit base.Such a greater surface area can increase adhesion between the mountingunit and the host's skin, minimize movement between the mounting unitand the host's skin, and/or protect the wound exit-site (sensorinsertion site) from environmental and/or biological contamination. Insome alternative embodiments, however, the adhesive pad can be smallerin surface area than the back surface assuming a sufficient adhesion canbe accomplished.

In some embodiments, the adhesive pad is substantially the same shape asthe back surface of the base, although other shapes can also beadvantageously employed, for example, butterfly-shaped, round, square,or rectangular. The adhesive pad backing can be designed for two-steprelease, for example, a primary release wherein only a portion of theadhesive pad is initially exposed to allow adjustable positioning of thedevice, and a secondary release wherein the remaining adhesive pad islater exposed to firmly and securely adhere the device to the host'sskin once appropriately positioned. The adhesive pad is preferablywaterproof. Preferably, a stretch-release adhesive pad is provided onthe back surface of the base portion to enable easy release from thehost's skin at the end of the useable life of the sensor.

In some circumstances, it has been found that a conventional bondbetween the adhesive pad and the mounting unit may not be sufficient,for example, due to humidity that can cause release of the adhesive padfrom the mounting unit. Accordingly, in some embodiments, the adhesivepad can be bonded using a bonding agent activated by or accelerated byan ultraviolet, acoustic, radio frequency, or humidity cure. In someembodiments, a eutectic bond of first and second composite materials canform a strong adhesion. In some embodiments, the surface of the mountingunit can be pretreated utilizing ozone, plasma, chemicals, or the like,in order to enhance the bondability of the surface.

A bioactive agent is preferably applied locally at the insertion siteprior to or during sensor insertion. Suitable bioactive agents includethose which are known to discourage or prevent bacterial growth andinfection, for example, anti-inflammatory agents, antimicrobials,antibiotics, or the like. It is believed that the diffusion or presenceof a bioactive agent can aid in prevention or elimination of bacteriaadjacent to the exit-site. Additionally or alternatively, the bioactiveagent can be integral with or coated on the adhesive pad, or nobioactive agent at all is employed.

In some embodiments, an applicator is provided for inserting the sensorthrough the host's skin at the appropriate insertion angle with the aidof a needle, and for subsequent removal of the needle using a continuouspush-pull action. Preferably, the applicator comprises an applicatorbody that guides the applicator and includes an applicator body baseconfigured to mate with the mounting unit during insertion of the sensorinto the host. The mate between the applicator body base and themounting unit can use any known mating configuration, for example, asnap-fit, a press-fit, an interference-fit, or the like, to discourageseparation during use. One or more release latches enable release of theapplicator body base, for example, when the applicator body base is snapfit into the mounting unit.

The sensor electronics includes hardware, firmware, and/or software thatenable measurement of levels of the analyte via the sensor. For example,the sensor electronics can comprise a potentiostat, a power source forproviding power to the sensor, other components useful for signalprocessing, and preferably an RF module for transmitting data from thesensor electronics to a receiver. Electronics can be affixed to aprinted circuit board (PCB), or the like, and can take a variety offorms. For example, the electronics can take the form of an integratedcircuit (IC), such as an Application-Specific Integrated Circuit (ASIC),a microcontroller, or a processor. Preferably, sensor electronicscomprise systems and methods for processing sensor analyte data.Examples of systems and methods for processing sensor analyte data aredescribed in more detail below and in co-pending U.S. application Ser.No. 10/633,367 filed Aug. 1, 2003, and entitled, “SYSTEM AND METHODS FORPROCESSING ANALYTE SENSOR DATA.”

In this embodiment, after insertion of the sensor using the applicator,and subsequent release of the applicator from the mounting unit, thesensor electronics are configured to releasably mate with the mountingunit. In one embodiment, the electronics are configured withprogramming, for example initialization, calibration reset, failuretesting, or the like, each time it is initially inserted into themounting unit and/or each time it initially communicates with thesensor.

Sensor Electronics

The following description of electronics associated with the sensor isapplicable to a variety of continuous analyte sensors, such asnon-invasive, minimally invasive, and/or invasive (e.g., transcutaneousand wholly implantable) sensors. For example, the sensor electronics anddata processing as well as the receiver electronics and data processingdescribed below can be incorporated into the wholly implantable glucosesensor disclosed in co-pending U.S. patent application Ser. No.10/838,912, filed May 3, 2004 and entitled “IMPLANTABLE ANALYTE SENSOR”and U.S. patent application Ser. No. 10/885,476 filed Jul. 6, 2004 andentitled, “SYSTEMS AND METHODS FOR MANUFACTURE OF AN ANALYTE-MEASURINGDEVICE INCLUDING A MEMBRANE SYSTEM”.

In one embodiment, a potentiostat, which is operably connected to anelectrode system (such as described above) provides a voltage to theelectrodes, which biases the sensor to enable measurement of an currentsignal indicative of the analyte concentration in the host (alsoreferred to as the analog portion). In some embodiments, thepotentiostat includes a resistor that translates the current intovoltage. In some alternative embodiments, a current to frequencyconverter is provided that is configured to continuously integrate themeasured current, for example, using a charge counting device. An A/Dconverter digitizes the analog signal into a digital signal, alsoreferred to as “counts” for processing. Accordingly, the resulting rawdata stream in counts, also referred to as raw sensor data, is directlyrelated to the current measured by the potentiostat.

A processor module includes the central control unit that controls theprocessing of the sensor electronics. In some embodiments, the processormodule includes a microprocessor, however a computer system other than amicroprocessor can be used to process data as described herein, forexample an ASIC can be used for some or all of the sensor's centralprocessing. The processor typically provides semi-permanent storage ofdata, for example, storing data such as sensor identifier (ID) andprogramming to process data streams (for example, programming for datasmoothing and/or replacement of signal artifacts such as is described inco-pending U.S. patent application Ser. No. 10/648,849, filed Aug. 22,2003, and entitled, “SYSTEMS AND METHODS FOR REPLACING SIGNAL ARTIFACTSIN A GLUCOSE SENSOR DATA STREAM”). The processor additionally can beused for the system's cache memory, for example for temporarily storingrecent sensor data. In some embodiments, the processor module comprisesmemory storage components such as ROM, RAM, dynamic-RAM, static-RAM,non-static RAM, EEPROM, rewritable ROMs, flash memory, or the like.

In some embodiments, the processor module comprises a digital filter,for example, an IIR or FIR filter, configured to smooth the raw datastream from the A/D converter. Generally, digital filters are programmedto filter data sampled at a predetermined time interval (also referredto as a sample rate). In some embodiments, wherein the potentiostat isconfigured to measure the analyte at discrete time intervals, these timeintervals determine the sample rate of the digital filter. In somealternative embodiments, wherein the potentiostat is configured tocontinuously measure the analyte, for example, using acurrent-to-frequency converter as described above, the processor modulecan be programmed to request a digital value from the A/D converter at apredetermined time interval, also referred to as the acquisition time.In these alternative embodiments, the values obtained by the processorare advantageously averaged over the acquisition time due the continuityof the current measurement. Accordingly, the acquisition time determinesthe sample rate of the digital filter. In preferred embodiments, theprocessor module is configured with a programmable acquisition time,namely, the predetermined time interval for requesting the digital valuefrom the A/D converter is programmable by a user within the digitalcircuitry of the processor module. An acquisition time of from about 2seconds to about 512 seconds is preferred; however any acquisition timecan be programmed into the processor module. A programmable acquisitiontime is advantageous in optimizing noise filtration, time lag, andprocessing/battery power.

Preferably, the processor module is configured to build the data packetfor transmission to an outside source, for example, an RF transmissionto a receiver as described in more detail below. Generally, the datapacket comprises a plurality of bits that can include a sensor ID code,raw data, filtered data, and/or error detection or correction. Theprocessor module can be configured to transmit any combination of rawand/or filtered data.

In some embodiments, the processor module further comprises atransmitter portion that determines the transmission interval of thesensor data to a receiver, or the like. In some embodiments, thetransmitter portion, which determines the interval of transmission, isconfigured to be programmable. In one such embodiment, a coefficient canbe chosen (e.g., a number of from about 1 to about 100, or more),wherein the coefficient is multiplied by the acquisition time (orsampling rate), such as described above, to define the transmissioninterval of the data packet. Thus, in some embodiments, the transmissioninterval is programmable between about 2 seconds and about 850 minutes,more preferably between about 30 second and 5 minutes; however, anytransmission interval can be programmable or programmed into theprocessor module. However, a variety of alternative systems and methodsfor providing a programmable transmission interval can also be employed.By providing a programmable transmission interval, data transmission canbe customized to meet a variety of design criteria (e.g., reducedbattery consumption, timeliness of reporting sensor values, etc.)

Conventional glucose sensors measure current in the nanoAmp range. Incontrast to conventional glucose sensors, the preferred embodiments areconfigured to measure the current flow in the picoAmp range, and in someembodiments, femtoAmps. Namely, for every unit (mg/dL) of glucosemeasured, at least one picoAmp of current is measured. Preferably, theanalog portion of the A/D converter is configured to continuouslymeasure the current flowing at the working electrode and to convert thecurrent measurement to digital values representative of the current. Inone embodiment, the current flow is measured by a charge counting device(e.g., a capacitor). Thus, a signal is provided, whereby a highsensitivity maximizes the signal received by a minimal amount ofmeasured hydrogen peroxide (e.g., minimal glucose requirements withoutsacrificing accuracy even in low glucose ranges), reducing thesensitivity to oxygen limitations in vivo (e.g., in oxygen-dependentglucose sensors).

A battery is operably connected to the sensor electronics and providesthe power for the sensor. In one embodiment, the battery is a lithiummanganese dioxide battery; however, any appropriately sized and poweredbattery can be used (for example, AAA, nickel-cadmium, zinc-carbon,alkaline, lithium, nickel-metal hydride, lithium-ion, zinc-air,zinc-mercury oxide, silver-zinc, and/or hermetically-sealed). In someembodiments, the battery is rechargeable, and/or a plurality ofbatteries can be used to power the system. The sensor can betranscutaneously powered via an inductive coupling, for example. In someembodiments, a quartz crystal is operably connected to the processor andmaintains system time for the computer system as a whole, for examplefor the programmable acquisition time within the processor module.

Optional temperature probe can be provided, wherein the temperatureprobe is located on the electronics assembly or the glucose sensoritself. The temperature probe can be used to measure ambient temperaturein the vicinity of the glucose sensor. This temperature measurement canbe used to add temperature compensation to the calculated glucose value.

An RF module is operably connected to the processor and transmits thesensor data from the sensor to a receiver within a wireless transmissionvia antenna. In some embodiments, a second quartz crystal provides thetime base for the RF carrier frequency used for data transmissions fromthe RF transceiver. In some alternative embodiments, however, othermechanisms, such as optical, infrared radiation (IR), ultrasonic, or thelike, can be used to transmit and/or receive data.

In the RF telemetry module of the preferred embodiments, the hardwareand software are designed for low power requirements to increase thelongevity of the device (for example, to enable a life of from about 3to about 24 months, or more) with maximum RF transmittance from the invivo environment to the ex vivo environment for wholly implantablesensors (for example, a distance of from about one to ten meters ormore). Preferably, a high frequency carrier signal of from about 402 MHzto about 433 MHz is employed in order to maintain lower powerrequirements. Additionally, in wholly implantable devices, the carrierfrequency is adapted for physiological attenuation levels, which isaccomplished by tuning the RF module in a simulated in vivo environmentto ensure RF functionality after implantation; accordingly, thepreferred glucose sensor can sustain sensor function for 3 months, 6months, 12 months, or 24 months or more.

In some embodiments, output signal (from the sensor electronics) is sentto a receiver (e.g., a computer or other communication station). Theoutput signal is typically a raw data stream that is used to provide auseful value of the measured analyte concentration to a patient or adoctor, for example. In some embodiments, the raw data stream can becontinuously or periodically algorithmically smoothed or otherwisemodified to diminish outlying points that do not accurately representthe analyte concentration, for example due to signal noise or othersignal artifacts, such as described in co-pending U.S. patentapplication Ser. No. 10/632,537 entitled, “SYSTEMS AND METHODS FORREPLACING SIGNAL ARTIFACTS IN A GLUCOSE SENSOR DATA STREAM,” filed Aug.22, 2003, which is incorporated herein by reference in its entirety.

When a sensor is first implanted into host tissue, the sensor andreceiver are initialized. This can be referred to as start-up mode, andinvolves optionally resetting the sensor data and calibrating thesensor. In selected embodiments, mating the electronics unit to themounting unit triggers a start-up mode. In other embodiments, thestart-up mode is triggered by the receiver.

Receiver

In some embodiments, the sensor electronics are wirelessly connected toa receiver via one- or two-way RF transmissions or the like. However, awired connection is also contemplated. The receiver provides much of theprocessing and display of the sensor data, and can be selectively wornand/or removed at the host's convenience. Thus, the sensor system can bediscreetly worn, and the receiver, which provides much of the processingand display of the sensor data, can be selectively worn and/or removedat the host's convenience. Particularly, the receiver includesprogramming for retrospectively and/or prospectively initiating acalibration, converting sensor data, updating the calibration,evaluating received reference and sensor data, and evaluating thecalibration for the analyte sensor, such as described in more detailwith reference to co-pending U.S. patent application Ser. No.10/633,367, filed Aug. 1, 2003 and entitled, “SYSTEM AND METHODS FORPROCESSING ANALYTE SENSOR DATA.”

FIG. 3C is a side schematic view of a wholly implantable analyte sensor58 in one embodiment. The sensor includes a sensor body 60 suitable forsubcutaneous implantation and includes a small structured sensor 34 asdefined herein. Published U.S. Patent Application No. 2004/0199059 toBrauker et al. describes systems and methods suitable for the sensorbody 60, and is incorporated herein by reference in its entirety. In thepreferred embodiments, a biointerface membrane 68 is formed onto thesensing mechanism 34 as described in more detail elsewhere herein. Thesensor body 60 includes sensor electronics and preferably communicateswith a receiver as described in more detail, above.

FIG. 3D is a side schematic view of a wholly implantable analyte sensor62 in an alternative embodiment. The sensor 62 includes a sensor body 60and a small structured sensor 34 as defined herein. The sensor body 60includes sensor electronics and preferably communicates with a receiveras described in more detail, above.

In preferred embodiments, a biointerface membrane 68 is formed onto thesensing mechanism 34 as described in more detail elsewhere herein.Preferably, a matrix or framework 64 surrounds the sensing mechanism 34for protecting the sensor from some foreign body processes, for example,by causing tissue to compress against or around the framework 64 ratherthan the sensing mechanism 34.

In general, the optional protective framework 64 is formed from atwo-dimensional or three-dimensional flexible, semi-rigid, or rigidmatrix (e.g., mesh), and which includes spaces or pores through whichthe analyte can pass. In some embodiments, the framework is incorporatedas a part of the biointerface membrane, however a separate framework canbe provided. While not wishing to be bound by theory, it is believedthat the framework 64 protects the small structured sensing mechanismfrom mechanical forces created in vivo.

FIG. 3E is a side schematic view of a wholly implantable analyte sensor66 in another alternative embodiment. The sensor 66 includes a sensorbody 60 and a small structured sensor 34, as defined herein, with abiointerface membrane 68 such as described in more detail elsewhereherein. Preferably, a framework 64 protects the sensing mechanism 34such as described in more detail above. The sensor body 60 includessensor electronics and preferably communicates with a receiver asdescribed in more detail, above.

In certain embodiments, the sensing device, which is adapted to bewholly implanted into the host, such as in the soft tissue beneath theskin, is implanted subcutaneously, such as in the abdomen of the host,for example. One skilled in the art appreciates a variety of suitableimplantation sites available due to the sensor's small size. In someembodiments, the sensor architecture is less than about 0.5 mm in atleast one dimension, for example a wire-based sensor with a diameter ofless than about 0.5 mm. In another exemplary embodiment, for example,the sensor may be 0.5 mm thick, 3 mm in length and 2 cm in width, suchas possibly a narrow substrate, needle, wire, rod, sheet, or pocket. Inanother exemplary embodiment, a plurality of about 1 mm wide wires about5 mm in length could be connected at their first ends, producing aforked sensor structure. In still another embodiment, a 1 mm wide sensorcould be coiled, to produce a planar, spiraled sensor structure.Although a few examples are cited above, numerous other usefulembodiments are contemplated by the present invention, as is appreciatedby one skilled in the art.

Post implantation, a period of time is allowed for tissue ingrowthwithin the biointerface. The length of time required for tissue ingrowthvaries from host to host, such as about a week to about 3 weeks,although other time periods are also possible. Once a mature bed ofvascularized tissue has grown into the biointerface, a signal can bedetected from the sensor, as described elsewhere herein and in U.S.patent application Ser. No. 10/838,912 to Brauker et al., entitledIMPLANTABLE ANALYTE SENSOR, incorporated herein in its entirety. Longterm sensors can remain implanted and produce glucose signal informationfrom months to years, as described in the above-cited patentapplication.

In certain embodiments, the device is configured such that the sensingunit is separated from the electronics unit by a tether or cable, or asimilar structure, similar to that illustrated in FIG. 3B. One skilledin the art will recognize that a variety of known and useful means maybe used to tether the sensor to the electronics. While not wishing to bebound by theory, it is believed that the FBR to the electronics unitalone may be greater than the FBR to the sensing unit alone, due to theelectronics unit's greater mass, for example. Accordingly, separation ofthe sensing and electronics units effectively reduces the FBR to thesensing unit and results in improved device function. As describedelsewhere herein, the architecture and/or composition of the sensingunit (e.g., inclusion of a biointerface with certain bioactive agents)can be implemented to further reduce the foreign body response to thetethered sensing unit.

In another embodiment, an analyte sensor is designed with separateelectronics and sensing units, wherein the sensing unit is inductivelycoupled to the electronics unit. In this embodiment, the electronicsunit provides power to the sensing unit and/or enables communication ofdata therebetween. FIGS. 3F and 3G illustrate exemplary systems thatemploy inductive coupling between an electronics unit 52 and a sensingunit 58.

FIG. 3F is a side view of one embodiment of an implanted sensorinductively coupled to an electronics unit within a functionally usefuldistance on the host's skin. FIG. 3F illustrates a sensing unit 58,including a sensing mechanism 34, biointerface 68 and small electronicschip 216 implanted below the host's skin 212, within the host's tissue210. In this example, the majority of the electronics associated withthe sensor are housed in an electronics unit 52 (also referred to as amounting unit) located within suitably close proximity on the host'sskin. The electronics unit 52 is inductively coupled to the smallelectronics chip 216 on the sensing unit 58 and thereby transmits powerto the sensor and/or collects data, for example. The small electronicschip 216 coupled to the sensing unit 58 provides the necessaryelectronics to provide a bias potential to the sensor, measure thesignal output, and/or other necessary requirements to allow the sensingmechanism 58 to function (e.g., chip 216 can include an ASIC(application specific integrated circuit), antenna, and other necessarycomponents appreciated by one skilled in the art).

In yet another embodiment, the implanted sensor additionally includes acapacitor to provide necessary power for device function. A portablescanner (e.g., wand-like device) is used to collect data stored on thecircuit and/or to recharge the device.

In general, inductive coupling, as described herein, enables power to betransmitted to the sensor for continuous power, recharging, and thelike. Additionally, inductive coupling utilizes appropriately spaced andoriented antennas (e.g., coils) on the sensing unit and the electronicsunit so as to efficiently transmit/receive power (e.g., current) and/ordata communication therebetween. One or more coils in each of thesensing and electronics unit can provide the necessary power inductionand/or data transmission.

In this embodiment, the sensing mechanism can be, for example, awire-based sensor as described in more detail with reference to FIGS. 2Aand 2B and as described in published U.S. Patent ApplicationUS2006-0020187, or a planar substrate-based sensor such as described inU.S. Pat. No. 6,175,752 to Say et al. and U.S. Pat. No. 5,779,665 toMastrototaro et al., all of which are incorporated herein by referencein their entirety. The biointerface 68 can be any suitable biointerfaceas described in more detail elsewhere herein, for example, a layer ofporous biointerface membrane material, a mesh cage, and the like. In oneexemplary embodiment, the biointerface 68 is a single- or multi-layersheet (e.g., pocket) of porous membrane material, such as ePTFE, inwhich the sensing mechanism 34 is incorporated.

FIG. 3G is a side view of on embodiment of an implanted sensorinductively coupled to an electronics unit implanted in the host'stissue at a functionally useful distance. FIG. 3G illustrates a sensorunit 58 and an electronics unit 52 similar to that described withreference to FIG. 3F, above, however both are implanted beneath thehost's skin in a suitably close proximity.

In general, it is believed that when the electronics unit 52, whichcarries the majority of the mass of the implantable device, is separatefrom the sensing unit 58, a lesser foreign body response will occursurrounding the sensing unit (e.g., as compared to a device of greatermass, for example, a device including certain electronics and/or powersupply). Thus, the configuration of the sensing unit, including abiointerface, can be optimized to minimize and/or modify the host'stissue response, for example with minimal mass as described in moredetail elsewhere.

Biointerface

In preferred embodiments, the sensor includes a porous material disposedover some portion thereof, which modifies the host's tissue response tothe sensor. In some embodiments, the porous material surrounding thesensor advantageously enhances and extends sensor performance andlifetime in the short term by slowing or reducing cellular migration tothe sensor and associated degradation that would otherwise be caused bycellular invasion if the sensor were directly exposed to the in vivoenvironment. Alternatively, the porous material can providestabilization of the sensor via tissue ingrowth into the porous materialin the long term. Suitable porous materials include silicone,polytetrafluoroethylene, expanded polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyolefin, polyester,polycarbonate, biostable polytetrafluoroethylene, homopolymers,copolymers, terpolymers of polyurethanes, polypropylene (PP),polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polyvinylalcohol (PVA), polybutylene terephthalate (PBT), polymethylmethacrylate(PMMA), polyether ether ketone (PEEK), polyamides, polyurethanes,cellulosic polymers, poly(ethylene oxide), poly(propylene oxide) andcopolymers and blends thereof, polysulfones and block copolymers thereofincluding, for example, di-block, tri-block, alternating, random andgraft copolymers, as well as metals, ceramics, cellulose, hydrogelpolymers, poly (2-hydroxyethyl methacrylate, pHEMA), hydroxyethylmethacrylate, (HEMA), polyacrylonitrile-polyvinyl chloride (PAN-PVC),high density polyethylene, acrylic copolymers, nylon, polyvinyldifluoride, polyanhydrides, poly(1-lysine), poly (L-lactic acid),hydroxyethylmetharcrylate, hydroxyapeptite, alumina, zirconia, carbonfiber, aluminum, calcium phosphate, titanium, titanium alloy, nintinol,stainless steel, and CoCr alloy, or the like, such as are described inco-pending U.S. patent application Ser. No. 10/842,716, filed May 10,2004 and entitled, “BIOINTERFACE MEMBRANES INCORPORATING BIOACTIVEAGENTS” and U.S. patent application Ser. No. 10/647,065 filed Aug. 22,2003 and entitled “POROUS MEMBRANES FOR USE WITH IMPLANTABLE DEVICES.”

In some embodiments, the porous material surrounding the sensor providesunique advantages in the short term (e.g., one to 14 days) that can beused to enhance and extend sensor performance and lifetime. However,such materials can also provide advantages in the long term too (e.g.,greater than 14 days). Particularly, the in vivo portion of the sensor(the portion of the sensor that is implanted into the host's tissue) isencased (partially or fully) in a porous material. The porous materialcan be wrapped around the sensor (for example, by wrapping the porousmaterial around the sensor or by inserting the sensor into a section ofporous material sized to receive the sensor). Alternately, the porousmaterial can be deposited on the sensor (for example, by electrospinningof a polymer directly thereon). In yet other alternative embodiments,the sensor is inserted into a selected section of porous biomaterial.Other methods for surrounding the in vivo portion of the sensor with aporous material can also be used as is appreciated by one skilled in theart.

The porous material surrounding the sensor advantageously slows orreduces cellular migration to the sensor and associated degradation thatwould otherwise be caused by cellular invasion if the sensor weredirectly exposed to the in vivo environment. Namely, the porous materialprovides a barrier that makes the migration of cells towards the sensormore tortuous and therefore slower (providing short term advantages). Itis believed that this reduces or slows the sensitivity loss normallyobserved in a short-term sensor over time.

In an embodiment wherein the porous material is a high oxygen solubilitymaterial, such as porous silicone, the high oxygen solubility porousmaterial surrounds some of or the entire in vivo portion of the sensor.In some embodiments, a lower ratio of oxygen-to-glucose can besufficient to provide excess oxygen by using a high oxygen solubledomain (for example, a silicone- or fluorocarbon-based material) toenhance the supply/transport of oxygen to the enzyme membrane and/orelectroactive surfaces. It is believed that some signal noise normallyseen by a conventional sensor can be attributed to an oxygen deficit.Silicone has high oxygen permeability, thus promoting oxygen transportto the enzyme layer. By enhancing the oxygen supply through the use of asilicone composition, for example, glucose concentration can be less ofa limiting factor. In other words, if more oxygen is supplied to theenzyme and/or electroactive surfaces, then more glucose can also besupplied to the enzyme without creating an oxygen rate-limiting excess.While not being bound by any particular theory, it is believed thatsilicone materials provide enhanced bio-stability when compared to otherpolymeric materials such as polyurethane.

In certain aspects, modifying a small structured sensor with abiointerface structure, material, matrix, and/or membrane that creates aspace appropriate for filling with fluid in vivo can enhance sensorperformance. In some embodiments, the small structured sensor includes aporous biointerface material, which allows fluid from the surroundingtissues to form a fluid-filled pocket around at least a portion of thesensor. It is believed that the fluid-filled pocket provides asufficient source of analyte-containing fluid for accurate sensormeasurement in the short term. Additionally or alternatively, inclusionof bioactive agents can modify the host's tissue response, for exampleto reduce or eliminate tissue ingrowth or other cellular responses intothe biointerface.

In some aspects, modifying a small structured sensor with a structure,material, and/or membrane/matrix that allows tissue ingrowth withoutbarrier cell formation can enhance sensor performance. For example, avascularized bed of tissue for long-term analyte sensor measurement. Insome embodiments, a porous biointerface membrane, including a pluralityof interconnected cavities and a solid portion, covering at least thesensing portion of a small structured sensor allows vascularized tissueingrowth therein. Vascularized tissue ingrowth provides a sufficientsource of analyte-containing tissue in the long term. Additionally oralternatively, inclusion of bioactive agents can modify the host'stissue response, for example to reduce or eliminate barrier cell layerformation within the membrane.

When used herein, the terms “membrane” and “matrix” are meant to beinterchangeable. In these embodiments first domain is provided thatincludes an architecture, including cavity size, configuration, and/oroverall thickness, that modifies the host's tissue response, forexample, by creating a fluid pocket, encouraging vascularized tissueingrowth, disrupting downward tissue contracture, resisting fibroustissue growth adjacent to the device, and/or discouraging barrier cellformation. The biointerface preferably covers at least the sensingmechanism of the sensor and can be of any shape or size, includinguniform, asymmetrically, or axi-symmetrically covering or surrounding asensing mechanism or sensor.

A second domain is optionally provided that is impermeable to cellsand/or cell processes. A bioactive agent is optionally provided that isincorporated into the at least one of the first domain, the seconddomain, the sensing membrane, or other part of the implantable device,wherein the bioactive agent is configured to modify a host tissueresponse.

FIG. 4A is a cross-sectional schematic view of a biointerface membrane70 in vivo in one exemplary embodiment, wherein the membrane comprises afirst domain 72 and an optional second domain 74. In the short term, thearchitecture of the biointerface membrane provides a space between thesensor and the host's tissue that allows a fluid filled pocket to formfor transport of fluid therein. In the long term, the architecture ofthe membrane provides a robust, implantable membrane that facilitatesthe transport of analytes through vascularized tissue ingrowth withoutthe formation of a barrier cell layer.

The first domain 72 comprises a solid portion 76 and a plurality ofinterconnected three-dimensional cavities 78 formed therein. In thisembodiment, the cavities 78 have sufficient size and structure to allowinvasive cells, such as fibroblasts 75, a fibrous matrix 77, and bloodvessels 79 to enter into the apertures 80 that define the entryway intoeach cavity 78, and to pass through the interconnected cavities towardthe interface 82 between the first and second domains. The cavitiescomprise an architecture that encourages the ingrowth of vascular tissuein vivo, as indicated by the blood vessels 79 formed throughout thecavities. Because of the vascularization within the cavities, solutes 73(for example, oxygen, glucose and other analytes) pass through the firstdomain with relative ease, and/or the diffusion distance (namely,distance that the glucose diffuses) is reduced.

Architecture of the First Domain

In some embodiments, the first domain of the biointerface membraneincludes an architecture that supports tissue ingrowth, disruptscontractile forces typically found in a foreign body response,encourages vascularity within the membrane, and disrupts the formationof a barrier cell layer. In some alternative embodiments, the firstdomain of the biointerface membrane includes an architecture thatcreates a fluid-filled space surrounding an implanted device, whichallows the passage of the analyte, but protects sensitive portions ofthe device from substantial fibrous tissue ingrowth and associatedforces.

In general, the first domain, also referred to as the cell disruptivedomain, comprises an open-celled configuration comprising interconnectedcavities and solid portions. The distribution of the solid portion andcavities of the first domain preferably includes a substantiallyco-continuous solid domain and includes more than one cavity in threedimensions substantially throughout the entirety of the first domain.However, some short-term embodiments may not require co-continuity ofthe cavities. Generally, cells can enter into the cavities; however,they cannot travel through or wholly exist within the solid portions.The cavities permit most substances to pass through, including, forexample, cells and molecules. One example of a suitable material isexpanded polytetrafluoraethylene (ePTFE).

Reference is now made to FIG. 4B, which is an illustration of themembrane of FIG. 4A, showing contractile forces caused by the fibroustissue in the long term (e.g., after about 3 weeks), for example, fromthe fibroblasts and fibrous matrix, of the FBR. Specifically, thearchitecture of the first domain, including the cavity interconnectivityand multiple-cavity depth, (namely, two or more cavities in threedimensions throughout a substantial portion of the first domain) canaffect the tissue contracture that typically occurs around a foreignbody.

The architecture of the first domain of the biointerface membrane,including the interconnected cavities and solid portion, is advantageousbecause the contractile forces caused by the downward tissue contracturethat can otherwise cause cells to flatten against the device and occludethe transport of analytes, is instead translated to, disrupted by,and/or counteracted by the forces 81 that contract around the solidportions 76 (for example, throughout the interconnected cavities 78)away from the device. That is, the architecture of the solid portions 76and cavities 78 of the first domain cause contractile forces 81 todisperse away from the interface between the first domain 72 and seconddomain 74. Without the organized contracture of fibrous tissue towardthe tissue-device interface 82 typically found in a FBC (FIG. 1),macrophages and foreign body giant cells do not form a substantialmonolayer of cohesive cells (namely, a barrier cell layer) and thereforethe transport of molecules across the second domain and/or membrane isnot blocked, as indicated by free transport of analyte 73 through thefirst and second domains in FIGS. 2A and 2B.

Various methods are suitable for use in manufacturing the first domainin order to create an architecture with preferred dimensions and overallstructure. The first domain can be manufactured by forming particles,for example, sugar granules, salt granules, and other natural orsynthetic uniform or non-uniform particles, in a mold, wherein theparticles have shapes and sizes substantially corresponding to thedesired cavity dimensions, such as described in more detail below. Insome methods, the particles are made to coalesce to provide the desiredinterconnectivity between the cavities. The desired material for thesolid portion can be introduced into the mold using methods common inthe art of polymer processing, for example, injecting, pressing,vacuuming, vapor depositing, pouring, and the like. After the solidportion material is cured or solidified, the coalesced particles arethen dissolved, melted, etched, or otherwise removed, leavinginterconnecting cavities within the solid portion. In such embodiments,sieving can be used to determine the dimensions of the particles, whichsubstantially correspond to the dimensions of resulting cavities. Insieving, also referred to as screening, the particles are added to thesieve and then shaken to produce overs and unders. The overs are theparticles that remain on the screen and the unders are the particlesthat pass through the screen. Other methods and apparatus known in theart are also suitable for use in determining particle size, for example,air classifiers, which apply opposing air flows and centrifugal forcesto separate particles having sizes down to 2 μm, can be used todetermine particle size when particles are smaller than 100 μm.

In one embodiment, the cavity size of the cavities 78 of the firstdomain is substantially defined by the particle size(s) used in creatingthe cavities. In some embodiments, the particles used to form thecavities can be substantially spherical, thus the dimensions belowdescribe a diameter of the particle and/or a diameter of the cavity. Insome alternative embodiments, the particles used to form the cavitiescan be non-spherical (for example, rectangular, square, diamond, orother geometric or non-geometric shapes), thus the dimensions belowdescribe one dimension (for example, shortest, average, or longest) ofthe particle and/or cavity.

In some embodiments, a variety of different particle sizes can be usedin the manufacture of the first domain. In some embodiments, thedimensions of the particles can be somewhat smaller or larger than thedimensions of the resulting cavities, due to dissolution or otherprecipitation that can occur during the manufacturing process.

Although one method of manufacturing porous domains is described above,a variety of methods known to one of ordinary skill in the art can beemployed to create the structures of preferred embodiments, see sectionentitled, “Formation of the Biointerface onto the Sensor,” below. Forexample, molds can be used in the place of the particles describedabove, such as coral, self-assembly beads, etched or broken siliconpieces, glass frit pieces, and the like. The dimensions of the mold candefine the cavity sizes, which can be determined by measuring thecavities of a model final product, and/or by other measuring techniquesknown in the art, for example, by a bubble point test. In U.S. Pat. No.3,929,971, Roy discloses a method of making a synthetic membrane havinga porous microstructure by converting calcium carbonate coral materialsto hydroxyapatite while at the same time retaining the uniquemicrostructure of the coral material.

Other methods of forming a three-dimensional first domain can be used,for example holographic lithography, stereolithography, and the like,wherein cavity sizes are defined and precisely formed by thelithographic or other such process to form a lattice of unit cells, asdescribed in co-pending U.S. patent application Ser. No. 11/055,779,entitled “Macro-Micro Architecture for Biointerface Membrane,” which isincorporated herein by reference in its entirety and as described byPekkarinen et al. in U.S. Pat. No. 6,520,997, which discloses aphotolithographic process for creating a porous membrane.

The first domain 72 can be defined using alternative methods. In analternative preferred embodiment, fibrous non-woven or woven materials,or other such materials, such as electrospun, felted, velvet, scattered,or aggregate materials, are manufactured by forming the solid portionswithout particularly defining the cavities therebetween. Accordingly, inthese alternative embodiments, structural elements that provide thethree-dimensional conformation can include fibers, strands, globules,cones, and/or rods of amorphous or uniform geometry. These elements arehereinafter referred to as “strands.” The solid portion of the firstdomain can include a plurality of strands, which generally defineapertures formed by a frame of the interconnected strands. The aperturesof the material form a framework of interconnected cavities. Formed inthis manner, the first domain is defined by a cavity size of about 0.6to about 1 mm in at least one dimension.

Referring to the dimensions and architecture of the first domain 72, theporous biointerface membranes can be loosely categorized into at leasttwo groups: those having a micro-architecture and those having amacro-architecture.

FIGS. 4A and 4B illustrate one preferred embodiment wherein thebiointerface membrane includes a macro-architecture as defined herein.In general, the cavity size of a macro-architecture provides aconfiguration and overall thickness that encourages vascular tissueingrowth and disrupts tissue contracture that is believed to causebarrier cell formation in the long term in vivo (as indicated by theblood vessels 79 formed throughout the cavities), while providing along-term, robust structure. Referring to the macro-architecture, asubstantial number of the cavities 78, defined using any of the methodsdescribed above, are greater than or equal to about 20 μm in onedimension. In some other embodiments, a substantial number of thecavities are greater than or equal to about 30, 40, 50, 60, 70, 80, 90,100, 120, 180, 160, 180, 200, 280, 280, 320, 360, 400, 500,600, 700 μm,and preferably less than about 1 mm in one dimension.

The biointerface membrane can also be formed with a micro-architectureas defined herein. Generally, at least some of the cavities of amicro-architecture have a sufficient size and structure to allowinflammatory cells to partially or completely enter into the cavities.However, in contrast to the macro-architecture, the micro-architecturedoes not allow extensive ingrowth of vascular and connective tissueswithin the cavities. Therefore, in some embodiments, themicro-architecture of preferred embodiments is defined by the actualsize of the cavity, wherein the cavities are formed from a mold, forexample, such as described in more detail above. However, in the contextof the micro-architecture it is preferable that the majority of the molddimensions, whether particles, beads, crystals, coral, self-assemblybeads, etched or broken silicon pieces, glass frit pieces, or other moldelements that form cavities, are less than about 20 μm in at least onedimension.

In some alternative embodiments, wherein the biointerface membrane isformed from a substantially fibrous material, the micro-architecture isdefined by a strand size of less than about 6 μm in all but the longestdimension, and a sufficient number of cavities are provided of a sizeand structure to allow inflammatory cells, for example, macrophages, tocompletely enter through the apertures that define the cavities, withoutextensive ingrowth of vascular and connective tissues.

In certain embodiments, the micro-architecture is characterized, ordefined, by standard pore size tests, such as the bubble point test. Themicro-architecture is selected with a nominal pore size of from about0.6 μm to about 20 μm. In some embodiments, the nominal pore size fromabout 1, 2, 3, 4, 5, 6, 7, 8, or 9 μm to about 10, 11, 12, 13, 14, 15,16, 17, 18, or 19 μm. It has been found that a porous polymer membranehaving an average nominal pore size of about 0.6 to about 20 μmfunctions satisfactorily in creating a vascular bed within themicro-architecture at the device-tissue interface. The term “nominalpore size” in the context of the micro-architecture in certainembodiments is derived from methods of analysis common to membrane, suchas the ability of the membrane to filter particles of a particular size,or the resistance of the membrane to the flow of fluids. Because of theamorphous, random, and irregular nature of most of these commerciallyavailable membranes, the “nominal pore size” designation may notactually indicate the size or shape of the apertures and cavities, whichin reality have a high degree of variability.

Accordingly, as used herein with reference to the micro-architecture,the term “nominal pore size” is a manufacturer's convention used toidentify a particular membrane of a particular commercial source whichhas a certain bubble point; as used herein, the term “pore” does notdescribe the size of the cavities of the material in the preferredembodiments. The bubble point measurement is described in PharmaceuticalTechnology, May 1983, pp. 76 to 82.

The optimum dimensions, architecture (for example, micro-architecture ormacro-architecture), and overall structural integrity of the membranecan be adjusted according to the parameters of the device that itsupports. For example, if the membrane is employed with aglucose-measuring device, the mechanical requirements of the membranecan be greater for devices having greater overall weight and surfacearea when compared to those that are relatively smaller.

In some embodiments, improved vascular tissue ingrowth in the long termis observed when the first domain has a thickness that accommodates adepth of at least two cavities throughout a substantial portion of thethickness. Improved vascularization results at least in part frommulti-layered interconnectivity of the cavities, such as in thepreferred embodiments, as compared to a surface topography such as seenin the prior art, for example, wherein the first domain has a depth ofonly one cavity throughout a substantial portion thereof. Themulti-layered interconnectivity of the cavities enables vascularizedtissue to grow into various layers of cavities in a manner that providesmechanical anchoring of the device with the surrounding tissue. Suchanchoring resists movement that can occur in vivo, which results inreduced sheer stress and scar tissue formation. The optimum depth ornumber of cavities can vary depending upon the parameters of the devicethat it supports. For example, if the membrane is employed with aglucose-measuring device, the anchoring that is required of the membraneis greater for devices having greater overall weight and surface area ascompared to those that are relatively smaller.

The thickness of the first domain can be optimized for decreasedtime-to-vascularize in vivo, that is, vascular tissue ingrowth can occursomewhat faster with a membrane that has a thin first domain as comparedto a membrane that has a relatively thicker first domain. Decreasedtime-to-vascularize results in faster stabilization and functionality ofthe biointerface in vivo. For example, in a subcutaneous implantableglucose device, consistent and increasing functionality of the device isat least in part a function of consistent and stable glucose transportacross the biointerface membrane, which is at least in part a functionof the vascularization thereof. Thus, quicker start-up time and/orshortened time lag (as when, for example, the diffusion path of theglucose through the membrane is reduced) can be achieved by decreasingthe thickness of the first domain.

The thickness of the first domain is typically from about 20 μm to about2000 μm, preferably from about 50, 60, 70, 80, 90, or 100 μm to about800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, or 1900μm, and most preferably from about 150, 200, 250, 300, 350, or 400 μm toabout 450, 500, 550, 600, 650, 700, or 750 μm. However, in somealternative embodiments a thinner or thicker cell disruptive domain(first domain) can be desired.

The solid portion preferably includes one or more materials such assilicone, polytetrafluoroethylene, expanded polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyolefin, polyester,polycarbonate, biostable polytetrafluoroethylene, homopolymers,copolymers, terpolymers of polyurethanes, polypropylene (PP),polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polyvinylalcohol (PVA), polybutylene terephthalate (PBT), polymethylmethacrylate(PMMA), polyether ether ketone (PEEK), polyamides, polyurethanes,cellulosic polymers, polysulfones and block copolymers thereofincluding, for example, di-block, tri-block, alternating, random andgraft copolymers. In some embodiments, the material selected for thefirst domain is an elastomeric material, for example, silicone, which isable to absorb stresses that can occur in vivo, such that sheer andother environmental forces are significantly minimized at the seconddomain. The solid portion can comprises a silicone composition with ahydrophile such as Polyethylene Glycol (PEG) covalently incorporated orgrafted therein, such as described in co-pending U.S. patent applicationSer. No. 10/695,676, filed Oct. 28, 2003, and entitled, “SILICONECOMPOSITION FOR BIOCOMPATIBLE MEMBRANE,” which is incorporated herein byreference in its entirety.

One preferred material that can be used to form the solid portion of thebiointerface matrix is a material that allows the passage of the analyte(e.g., glucose) there through. For example, the biointerface matrix maybe formed from a silicone polymer/hydrophobic-hydrophilic polymer blend.In one embodiment, The hydrophobic-hydrophilic polymer for use in theblend may be any suitable hydrophobic-hydrophilic polymer, including butnot limited to components such as polyvinylpyrrolidone (PVP),polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid,polyethers such as polyethylene glycol or polypropylene oxide, andcopolymers thereof, including, for example, di-block, tri-block,alternating, random, comb, star, dendritic, and graft copolymers (blockcopolymers are discussed in U.S. Pat. Nos. 4,803,243 and 4,686,044,which are incorporated herein by reference). In one embodiment, thehydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide)(PEO) and poly(propylene oxide) (PPO). Suitable such polymers include,but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblockcopolymers, PEO-PPO-PEO triblock copolymers, alternating blockcopolymers of PEO-PPO, random copolymers of ethylene oxide and propyleneoxide, and blends thereof. In some embodiments, the copolymers may beoptionally substituted with hydroxy substituents. Commercially availableexamples of PEO and PPO copolymers include the PLURONIC® brand ofpolymers available from BASF®. In one embodiment, PLURONIC® F-127 isused. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers(e.g., PLURONIC® R products). Other suitable commercial polymersinclude, but are not limited to, SYNPERONICS® products available fromUNIQEMA®.

The silicone polymer for use in the silicone/hydrophobic-hydrophilicpolymer blend may be any suitable silicone polymer. In some embodiments,the silicone polymer is a liquid silicone rubber that may be vulcanizedusing a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, orother radiation-catalyzed process. In some embodiments, the siliconepolymer is a dimethyl- and methylhydrogen-siloxane copolymer. In someembodiments, the copolymer has vinyl substituents. In some embodiments,commercially available silicone polymers may be used. For example,commercially available silicone polymer precursor compositions may beused to prepare the blends, such as described below. In one embodiment,MED-4840 available from NUSIL® Technology LLC is used as a precursor tothe silicone polymer used in the blend. MED-4840 consists of a 2-partsilicone elastomer precursor including vinyl-functionalized dimethyl-and methylhydrogen-siloxane copolymers, amorphous silica, a platinumcatalyst, a crosslinker, and an inhibitor. The two components may bemixed together and heated to initiate vulcanization, thereby forming anelastomeric solid material. Other suitable silicone polymer precursorsystems include, but are not limited to, MED-2174 peroxide-cured liquidsilicone rubber available from NUSIL® Technology LLC, SILASTIC®MDX4-4210 platinum-cured biomedical grade elastomer available from DOWCORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50)available from Applied Silicone Corporation.

Silicone polymer/hydrophobic-hydrophilic polymer blends are described inmore detail in U.S. patent application Ser. No. 11/404,417, entitled“SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filedon Apr. 14, 2006.

Additionally, elastomeric materials with a memory of the originalconfiguration can withstand greater stresses without affecting theconfiguration, and thus the function, of the device.

In some embodiments, the first domain can include a macro-architectureand a micro-architecture located within at least a portion of themacro-architecture, such as is described in co-pending U.S. patentapplication Ser. No. 11/055,779, entitled, “BIOINTERFACE WITH MACRO- ANDMICRO-ARCHITECTURE.” For example, the macro-architecture includes aporous structure with interconnected cavities such as described withreference to the solid portion of the first domain, wherein at leastsome portion of the cavities of the first domain are filled with themicro-architecture that includes a fibrous or other fine structuredmaterial that aids in preventing formation of a barrier cell layer, forexample in pockets in the bottom of the cavities of themacro-architecture adjacent to the implantable device.

In certain embodiments, other non-resorbable implant materials can beused in forming the first domain, including but not limited to, metals,ceramics, cellulose, hydrogel polymers, poly (2-hydroxyethylmethacrylate, pHEMA), hydroxyethyl methacrylate, (HEMA),polyacrylonitrile-polyvinyl chloride (PAN-PVC), high densitypolyethylene, acrylic copolymers, nylon, polyvinyl difluoride,polyanhydrides, poly(1-lysine), poly (L-lactic acid),hydroxyethylmetharcrylate, hydoxyapaptite, alumina, zirconia, carbonfiber, aluminum, calcium phosphate (and its chemical variants),titanium, titanium alloy, nintinol, stainless steel, and CoCr alloy.

Architecture of the Second Domain

FIGS. 4A and 4B, illustrate the optional second domain of the membrane.The second domain is impermeable to cells or cell processes, and iscomposed of a biostable material. In one exemplary embodiment, thesecond domain is comprised of polyurethane and a hydrophilic polymer,such as is described in U.S. Pat. No. 6,862,465 to Shults et al., whichis incorporated herein by reference in its entirety. Alternatively, theoutermost layer of the sensing membrane 32 can function as a cellimpermeable domain and therefore a second domain may not be a discretecomponent of the biointerface membrane.

In general, the materials preferred for the second domain prevent orhinder cell entry or contact with device elements underlying themembrane and prevent or hinder the adherence of cells, thereby furtherdiscouraging formation of a barrier cell layer. Additionally, because ofthe resistance of the materials to barrier cell layer formation,membranes prepared therefrom are robust long-term in vivo.

The thickness of the cell impermeable biomaterial of the second domain(also referred to as a cell impermeable domain) is typically about 1 μmor more, preferably from about 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, or50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150,160, 170, 180, 190, or 200 μm to about 500, 600, 700, 800, 900, or 1000μm. In some embodiments, thicker or thinner cell impermeable domains canbe desired. Alternatively, the function of the cell impermeable domainis accomplished by the implantable device, or a portion of theimplantable device, which may or may not include a distinct domain orlayer.

The characteristics of the cell impermeable membrane prevent or hindercells from entering the membrane, but permit or facilitate transport ofthe analyte of interest or a substance indicative of the concentrationor presence of the analyte. Additionally the second domain, similar tothe first domain, is preferably constructed of a biodurable material(for example, a material durable for a period of several years in vivo)that is impermeable to host cells, for example, macrophages, such asdescribed above.

In embodiments wherein the biointerface membrane is employed in animplantable glucose-measuring device, the biointerface membrane ispermeable to oxygen and glucose or a substance indicative of theconcentration of glucose. In embodiments wherein the membrane isemployed in a drug delivery device or other device for delivering asubstance to the body, the cell impermeable membrane is permeable to thedrug or other substance dispensed from the device. In embodimentswherein the membrane is employed for cell transplantation, the membraneis semi-permeable, for example, impermeable to immune cells and solublefactors responsible for rejecting transplanted tissue, but permeable tothe ingress of glucose and oxygen for the purpose of sustaining thetransplanted tissue; additionally, the second domain is permeable to theegress of the gene product of interest (for example, insulin).

The cell disruptive (first) domain and the cell impermeable (second)domain can be secured to each other by any suitable method as is knownin the art. For example, the cell impermeable domain can simply belayered or cast upon the porous cell disruptive domain so as to form amechanical attachment. Alternatively, chemical and/or mechanicalattachment methods can be suitable for use. Chemical attachment methodscan include adhesives, glues, lamination, and/or wherein a thermal bondis formed through the application of heat and pressure, and the like.Suitable adhesives are those capable of forming a bond between thematerials that make up both the barrier cell disruptive domain and thecell impermeable domain, and include liquid and/or film appliedadhesives. An appropriate material can be designed that can be used forpreparing both domains such that the composite is prepared in one step,thereby forming a unitary structure. For example, when the celldisruptive domain and the cell impermeable domain comprise silicone, thematerials can be designed so that they can be covalently cured to oneanother. However, in some embodiments wherein the second domaincomprises a part of the implantable device, it can be attached to orsimply lie adjacent to the first domain.

In some embodiments wherein an adhesive is employed, the adhesive cancomprise a biocompatible material. However, in some embodimentsadhesives not generally considered to have a high degree ofbiocompatibility can also be employed. Adhesives with varying degrees ofbiocompatibility suitable for use include acrylates, for example,cyanoacrylates, epoxies, methacrylates, polyurethanes, and otherpolymers, resins, RTV silicone, and crosslinking agents as are known inthe art. In some embodiments, a layer of woven or non-woven material(such as ePTFE) is cured to the first domain after which the material isbonded to the second domain, which allows a good adhesive interfacebetween the first and second domains using a biomaterial known torespond well at the tissue-device interface, for example.

Bioactive Agents

In some alternative embodiment, the biointerface membranes include abioactive agent, which is incorporated into at least one of the firstand second domains 72, 74 of the biointerface membrane, or which isincorporated into the device (e.g., sensing membrane 32) and adapted todiffuse through the first and/or second domains, in order to modify thetissue response of the host to the membrane. The architectures of thefirst and second domains have been shown to create a fluid pocket,support vascularized tissue ingrowth, to interfere with and resistbarrier cell layer formation, and to facilitate the transport ofanalytes across the membrane. However, the bioactive agent can furtherenhance formation of a fluid pocket, alter or enhance vascularizedtissue ingrowth, resistance to barrier cell layer formation, and therebyfacilitate the passage of analytes 73 across the device-tissue interface82.

In embodiments wherein the biointerface includes a bioactive agent, thebioactive agent is incorporated into at least one of the first andsecond domains of the biointerface membrane, or into the device andadapted to diffuse through the first and/or second domains, in order tomodify the tissue response of the host to the membrane. In general, thearchitectures of the first and second domains support vascularizedtissue growth in or around the biointerface membrane, interfere with andresist barrier cell layer formation, and/or allow the transport ofanalytes across the membrane. However, certain outside influences, forexample, faulty surgical techniques, acute or chronic movement of theimplant, or other surgery-, host-, and/or implantation site-relatedconditions, can create acute and/or chronic inflammation at the implantsite. When this occurs, the biointerface membrane architecture alone maynot be sufficient to overcome the acute and/or chronic inflammation.Alternatively, the membrane architecture can benefit from additionalmechanisms that aid in reducing this acute and/or chronic inflammationthat can produce a barrier cell layer and/or a fibrotic capsulesurrounding the implant, resulting in compromised solute transportthrough the membrane.

In general, the inflammatory response to biomaterial implants can bedivided into two phases. The first phase consists of mobilization ofmast cells and then infiltration of predominantly polymorphonuclear(PMN) cells. This phase is termed the acute inflammatory phase. Over thecourse of days to weeks, chronic cell types that comprise the secondphase of inflammation replace the PMNs. Macrophage and lymphocyte cellspredominate during this phase. While not wishing to be bound by anyparticular theory, it is believed that short-term stimulation ofvascularization, or short-term inhibition of scar formation or barriercell layer formation, provides protection from scar tissue formation,thereby providing a stable platform for sustained maintenance of thealtered foreign body response, for example.

Accordingly, bioactive intervention can modify the foreign body responsein the early weeks of foreign body capsule formation and alter thelong-term behavior of the foreign body capsule. Additionally, it isbelieved that in some circumstances the biointerface membranes of thepreferred embodiments can benefit from bioactive intervention toovercome sensitivity of the membrane to implant procedure, motion of theimplant, or other factors, which are known to otherwise causeinflammation, scar formation, and hinder device function in vivo.

In general, bioactive agents that are believed to modify tissue responseinclude anti-inflammatory agents, anti-infective agents, anesthetics,inflammatory agents, growth factors, angiogenic (growth) factors,adjuvants, immunosuppressive agents, antiplatelet agents,anticoagulants, ACE inhibitors, cytotoxic agents, anti-barrier cellcompounds, vascularization compounds, anti-sense molecules, and thelike. In some embodiments, preferred bioactive agents include S1P(Sphingosine-1-phosphate), Monobutyrin, Cyclosporin A,Anti-thrombospondin-2, Rapamycin (and its derivatives), andDexamethasone. However, other bioactive agents, biological materials(for example, proteins), or even non-bioactive substances canincorporated into the membranes of preferred embodiments.

Bioactive agents suitable for use in the preferred embodiments areloosely organized into two groups: anti-barrier cell agents andvascularization agents. These designations reflect functions that arebelieved to provide short-term solute transport through the biointerfacemembrane, and additionally extend the life of a healthy vascular bed andhence solute transport through the biointerface membrane long term invivo. However, not all bioactive agents can be clearly categorized intoone or other of the above groups; rather, bioactive agents generallycomprise one or more varying mechanisms for modifying tissue responseand can be generally categorized into one or both of the above-citedcategories.

Anti-Barrier Cell Agents

Generally, anti-barrier cell agents include compounds exhibiting affectson macrophages and foreign body giant cells (FBGCs). It is believed thatanti-barrier cell agents prevent closure of the barrier to solutetransport presented by macrophages and FBGCs at the device-tissueinterface during FBC maturation.

Anti-barrier cell agents generally include mechanisms that inhibitforeign body giant cells and/or occlusive cell layers. For example,Super Oxide Dismutase (SOD) Mimetic, which utilizes a manganesecatalytic center within a porphyrin like molecule to mimic native SODand effectively remove superoxide for long periods, thereby inhibitingFBGC formation at the surfaces of biomaterials in vivo, is incorporatedinto a biointerface membrane of a preferred embodiment.

Anti-barrier cell agents can include anti-inflammatory and/orimmunosuppressive mechanisms that affect early FBC formation.Cyclosporine, which stimulates very high levels of neovascularizationaround biomaterials, can be incorporated into a biointerface membrane ofa preferred embodiment (see U.S. Pat. No. 5,569,462 to Martinson etal.). Alternatively, Dexamethasone, which abates the intensity of theFBC response at the tissue-device interface, can be incorporated into abiointerface membrane of a preferred embodiment. Alternatively,Rapamycin, which is a potent specific inhibitor of some macrophageinflammatory functions, can be incorporated into a biointerface membraneof a preferred embodiment.

Other suitable medicaments, pharmaceutical compositions, therapeuticagents, or other desirable substances can be incorporated into themembranes of preferred embodiments, including, but not limited to,anti-inflammatory agents, anti-infective agents, necrosing agents, andanesthetics.

Generally, anti-inflammatory agents reduce acute and/or chronicinflammation adjacent to the implant, in order to decrease the formationof a FBC capsule to reduce or prevent barrier cell layer formation.Suitable anti-inflammatory agents include but are not limited to, forexample, nonsteroidal anti-inflammatory drugs (NSAIDs) such asacetometaphen, aminosalicylic acid, aspirin, celecoxib, cholinemagnesium trisalicylate, diclofenac potassium, diclofenac sodium,diflunisal, etodolac, fenoprofen, flurbiprofen, ibuprofen, indomethacin,interleukin (IL)-10, IL-6 mutein, anti-IL-6 iNOS inhibitors (forexample, L-NAME or L-NMDA), Interferon, ketoprofen, ketorolac,leflunomide, melenamic acid, mycophenolic acid, mizoribine, nabumetone,naproxen, naproxen sodium, oxaprozin, piroxicam, rofecoxib, salsalate,sulindac, and tolmetin; and corticosteroids such as cortisone,hydrocortisone, methylprednisolone, prednisone, prednisolone,betamethesone, beclomethasone dipropionate, budesonide, dexamethasonesodium phosphate, flunisolide, fluticasone propionate, paclitaxel,tacrolimus, tranilast, triamcinolone acetonide, betamethasone,fluocinolone, fluocinonide, betamethasone dipropionate, betamethasonevalerate, desonide, desoximetasone, fluocinolone, triamcinolone,triamcinolone acetonide, clobetasol propionate, and dexamethasone.

Generally, immunosuppressive and/or immunomodulatory agents interferedirectly with several key mechanisms necessary for involvement ofdifferent cellular elements in the inflammatory response. Suitableimmunosuppressive and/or immunomodulatory agents includeanti-proliferative, cell-cycle inhibitors, (for example, paclitaxol(e.g., Sirolimus), cytochalasin D, infiximab), taxol, actinomycin,mitomycin, thospromote VEGF, estradiols, NO donors, QP-2, tacrolimus,tranilast, actinomycin, everolimus, methothrexate, mycophenolic acid,angiopeptin, vincristing, mitomycine, statins, C MYC antisense,sirolimus (and analogs), RestenASE, 2-chloro-deoxyadenosine, PCNARibozyme, batimstat, prolyl hydroxylase inhibitors, PPARγ ligands (forexample troglitazone, rosiglitazone, pioglitazone), halofuginone,C-proteinase inhibitors, probucol, BCP671, EPC antibodies, catchins,glycating agents, endothelin inhibitors (for example, Ambrisentan,Tesosentan, Bosentan), Statins (for example, Cerivasttin), E. coliheat-labile enterotoxin, and advanced coatings.

Generally, anti-infective agents are substances capable of actingagainst infection by inhibiting the spread of an infectious agent or bykilling the infectious agent outright, which can serve to reduceimmuno-response without inflammatory response at the implant site.Anti-infective agents include, but are not limited to, anthelmintics(mebendazole), antibiotics including aminoclycosides (gentamicin,neomycin, tobramycin), antifungal antibiotics (amphotericin b,fluconazole, griseofulvin, itraconazole, ketoconazole, nystatin,micatin, tolnaftate), cephalosporins (cefaclor, cefazolin, cefotaxime,ceftazidime, ceftriaxone, cefuroxime, cephalexin), beta-lactamantibiotics (cefotetan, meropenem), chloramphenicol, macrolides(azithromycin, clarithromycin, erythromycin), penicillins (penicillin Gsodium salt, amoxicillin, ampicillin, dicloxacillin, nafcillin,piperacillin, ticarcillin), tetracyclines (doxycycline, minocycline,tetracycline), bacitracin; clindamycin; colistimethate sodium; polymyxinb sulfate; vancomycin; antivirals including acyclovir, amantadine,didanosine, efavirenz, foscarnet, ganciclovir, indinavir, lamivudine,nelfinavir, ritonavir, saquinavir, silver, stavudine, valacyclovir,valganciclovir, zidovudine; quinolones (ciprofloxacin, levofloxacin);sulfonamides (sulfadiazine, sulfisoxazole); sulfones (dapsone);furazolidone; metronidazole; pentamidine; sulfanilamidum crystallinum;gatifloxacin; and sulfamethoxazole/trimethoprim.

Generally, necrosing agents are any drug that causes tissue necrosis orcell death. Necrosing agents include cisplatin, BCNU, taxol or taxolderivatives, and the like.

Vascularization Agents

Generally, vascularization agents include substances with direct orindirect angiogenic properties. In some cases, vascularization agentsmay additionally affect formation of barrier cells in vivo. By indirectangiogenesis, it is meant that the angiogenesis can be mediated throughinflammatory or immune stimulatory pathways. It is not fully known howagents that induce local vascularization indirectly inhibit barrier-cellformation; however it is believed that some barrier-cell effects canresult indirectly from the effects of vascularization agents.

Vascularization agents include mechanisms that promoteneovascularization around the membrane and/or minimize periods ofischemia by increasing vascularization close to the tissue-deviceinterface. Sphingosine-1-Phosphate (S1P), which is a phospholipidpossessing potent angiogenic activity, is incorporated into abiointerface membrane of a preferred embodiment. Monobutyrin, which is apotent vasodilator and angiogenic lipid product of adipocytes, isincorporated into a biointerface membrane of a preferred embodiment. Inanother embodiment, an anti-sense molecule (for example,thrombospondin-2 anti-sense), which increases vascularization, isincorporated into a biointerface membrane.

Vascularization agents can include mechanisms that promote inflammation,which is believed to cause accelerated neovascularization in vivo. Inone embodiment, a xenogenic carrier, for example, bovine collagen, whichby its foreign nature invokes an immune response, stimulatesneovascularization, and is incorporated into a biointerface membrane ofthe preferred embodiments. In another embodiment, Lipopolysaccharide,which is a potent immunostimulant, is incorporated into a biointerfacemembrane. In another embodiment, a protein, for example, a bonemorphogenetic protein (BMP), which is known to modulate bone healing intissue, is incorporated into a biointerface membrane of a preferredembodiment.

Generally, angiogenic agents are substances capable of stimulatingneovascularization, which can accelerate and sustain the development ofa vascularized tissue bed at the tissue-device interface. Angiogenicagents include, but are not limited to, copper ions, iron ions,tridodecylmethylammonium chloride, Basic Fibroblast Growth Factor(bFGF), (also known as Heparin Binding Growth Factor-II and FibroblastGrowth Factor II), Acidic Fibroblast Growth Factor (aFGF), (also knownas Heparin Binding Growth Factor-I and Fibroblast Growth Factor-I),Vascular Endothelial Growth Factor (VEGF), Platelet Derived EndothelialCell Growth Factor BB (PDEGF-BB), Angiopoietin-1, Transforming GrowthFactor Beta (TGF-Beta), Transforming Growth Factor Alpha (TGF-Alpha),Hepatocyte Growth Factor, Tumor Necrosis Factor-Alpha (TNF-Alpha),Placental Growth Factor (PLGF), Angiogenin, Interleukin-8 (IL-8),Hypoxia Inducible Factor-I (HIF-1), Angiotensin-Converting Enzyme (ACE)Inhibitor Quinaprilat, Angiotropin, Thrombospondin, Peptide KGHK, LowOxygen Tension, Lactic Acid, Insulin, Copper Sulphate, Estradiol,prostaglandins, cox inhibitors, endothelial cell binding agents (forexample, decorin or vimentin), glenipin, hydrogen peroxide, nicotine,and Growth Hormone.

Generally, pro-inflammatory agents are substances capable of stimulatingan immune response in host tissue, which can accelerate or sustainformation of a mature vascularized tissue bed. For example,pro-inflammatory agents are generally irritants or other substances thatinduce chronic inflammation and chronic granular response at theimplantation-site. While not wishing to be bound by theory, it isbelieved that formation of high tissue granulation induces bloodvessels, which supply an adequate or rich supply of analytes to thedevice-tissue interface. Pro-inflammatory agents include, but are notlimited to, xenogenic carriers, Lipopolysaccharides, S. aureuspeptidoglycan, and proteins.

Other substances that can be incorporated into membranes of preferredembodiments include various pharmacological agents, excipients, andother substances well known in the art of pharmaceutical formulations.

U.S. Publication No. 2005/0031689 A1 to Shults et al. discloses avariety of systems and methods by which the bioactive agent can beincorporated into the biointerface membranes and/or implantable device.Although the bioactive agent is preferably incorporated into thebiointerface membrane and/or implantable device, in some embodiments thebioactive agent can be administered concurrently with, prior to, orafter implantation of the device systemically, for example, by oraladministration, or locally, for example, by subcutaneous injection nearthe implantation site. A combination of bioactive agent incorporated inthe biointerface membrane and bioactive agent administration locallyand/or systemically can be preferred in certain embodiments.

Generally, numerous variables can affect the pharmacokinetics ofbioactive agent release. The bioactive agents of the preferredembodiments can be optimized for short- and/or long-term release. Insome embodiments, the bioactive agents of the preferred embodiments aredesigned to aid or overcome factors associated with short-term effects(for example, acute inflammation) of the foreign body response, whichcan begin as early as the time of implantation and extend up to aboutone month after implantation. In some embodiments, the bioactive agentsof the preferred embodiments are designed to aid or overcome factorsassociated with long-term effects, for example, chronic inflammation,barrier cell layer formation, or build-up of fibrotic tissue of theforeign body response, which can begin as early as about one week afterimplantation and extend for the life of the implant, for example, monthsto years. In some embodiments, the bioactive agents of the preferredembodiments combine short- and long-term release to exploit the benefitsof both. Published U.S. Publication No. 2005/0031689 A1 to Shults et al.discloses a variety of systems and methods for release of the bioactiveagents.

The amount of loading of the bioactive agent into the biointerfacemembrane can depend upon several factors. For example, the bioactiveagent dosage and duration can vary with the intended use of thebiointerface membrane, for example, cell transplantation, analytemeasuring-device, and the like; differences among hosts in the effectivedose of bioactive agent; location and methods of loading the bioactiveagent; and release rates associated with bioactive agents and optionallytheir carrier matrix. Therefore, one skilled in the art will appreciatethe variability in the levels of loading the bioactive agent, for thereasons described above. U.S. Publication No. 2005/0031689 A1 to Shultset al. discloses a variety of systems and methods for loading of thebioactive agents.

Biointerface Membrane Formation onto the Sensor

Due to the small dimension(s) of the sensor (sensing mechanism) of thepreferred embodiments, some conventional methods of porous membraneformation and/or porous membrane adhesion are inappropriate for theformation of the biointerface membrane onto the sensor as describedherein. Accordingly, the following embodiments exemplify systems andmethods for forming and/or adhering a biointerface membrane onto a smallstructured sensor as defined herein. For example, the biointerfacemembrane of the preferred embodiments can be formed onto the sensorusing techniques such as electrospinning, molding, weaving,direct-writing, lyophilizing, wrapping, and the like.

Although FIGS. 5 to 9 describe systems and methods for the formation ofporous biointerface membranes, including interconnected cavities andsolid portion(s). In some embodiments, a cell impermeable (seconddomain) can additionally be formed using known thin film techniques,such as dip coating, spray coating, spin coating, tampo printing, andthe like, prior to formation of the interconnected cavities and solidportion(s). Alternatively, the porous biointerface membrane (e.g., firstdomain) can be formed directly onto the sensing membrane.

FIG. 5 is a flow chart that illustrates the process 150 of forming abiointerface-coated small structured sensor in one embodiment. In thisembodiment, the biointerface membrane includes woven or non-woven fibersformed directly onto the sensor. Generally, fibers can be deposited ontothe sensor using methods suitable for formation of woven- or non-wovenfibrous materials. In some embodiments, the biointerface membrane iselectrospun directly onto the sensor; electrospinning advantageouslyallows the biointerface membranes to be made with small consistent fiberdiameters that are fused at the nodes and are without aggregation.

In some embodiments, the biointerface membrane is directly written ontothe sensor; direct-writing can advantageously allow uniform depositionof stored patterns (e.g., in a computer system) for providing consistentand reproducible architectures. In these embodiments, a curing step isincluded either during or after the writing step to solidify thematerial being written (e.g., heat, UV curing, radiation, etc.).Direct-writing is described in more detail, below.

At block 152, one or more dispensers dispense a polymeric material usedto form the fibers. A variety of polymeric materials are contemplatedfor use with the preferred embodiments, including one or more ofsilicone, polytetrafluoroethylene, expanded polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyolefin, polyester,polycarbonate, biostable polytetrafluoroethylene, homopolymers,copolymers, terpolymers of polyurethanes, polypropylene (PP),polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polyvinylalcohol (PVA), polybutylene terephthalate (PBT), polymethylmethacrylate(PMMA), polyether ether ketone (PEEK), polyamides, polyurethanes,cellulosic polymers, poly(ethylene oxide), poly(propylene oxide) andcopolymers and blends thereof, polysulfones and block copolymers thereofincluding, for example, di-block, tri-block, alternating, random andgraft copolymers.

The coating process can be performed in a vacuum or in a gaseous medium,which environment may affect the architecture of the biointerfacemembrane as is appreciated by one skilled in the art.

In embodiments wherein the biointerface is electrospun onto the sensor,the dispenser dispenses a charged liquefied polymer within an electricfield, to thereby form a jet of polymer fibers, for example, such asdescribed in PCT International Publication No. WO 2005/032400 A1.

In embodiments wherein the biointerface is directly-written onto thesensor, a dispenser dispenses a polymer solution using a nozzle with avalve, or the like, for example as described in U.S. Publication No.2004/0253365 A1. In general, a variety of nozzles and/or dispensers canbe used to dispense a polymeric material to form the woven or non-wovenfibers of the biointerface membrane.

In general, a direct-write patterning system is suitable for eitherfine-pattern micro-dispensing and/or fine-focused laser-beam writingover flat or conformal surfaces to create exact replicas of a preferredbiointerface structure. In certain embodiments, the biointerfacematerials described herein may be deposited using these integrated tooltechnologies for the direct-write deposition and laser micromachining ofa wide variety of biointerface architectures described herein.Additionally, the direct-write patterning system can provide thecapability for concurrent detection and imaging methods during additiveand subtractive processes.

In another aspect, alternative embodiments of the direct-writingdeposition technique utilize a tool in which constituent materials maybe dispensed through multiple, discrete dispensing heads. In yet anotheralternative embodiment, the biointerface structure is directly-writtenonto a removable substrate, after which the substrate is removed and thebiointerface applied to the sensor (e.g., wrapped around the sensor orthe sensor is inserted into the biointerface).

At block 154, the dispenser(s) is moved relative to the sensor and/orthe sensor is moved relative to the dispenser(s) so as to coat thesensor with the fibers. In embodiments wherein the biointerface membraneis electrospun onto the sensor, the dispenser(s) can change thedirection and/or magnitude of the electric field during motion in orderto effect the orientation of the polymer fibers on the sensor.Additionally, the path of the dispenser is preferably selected so as tocoat the portions of or the entire object. In one exemplary embodiment,wherein it is desirable for the biointerface membrane to substantiallycircumscribe the sensor (e.g., a substantially cylindrical shape), suchas illustrated in FIG. 2A, the dispenser can be moved along a helixpath, a circular path, a zigzag path, or the like. Additionally, thedispenser can move rotationally and/or translationally relative to thesensor. The number of sweeps is preferably selected according to thedesired architecture of the biointerface membrane. Additionally, thedensity of the fibers and/or the type of liquefied polymer can bechanged from one sweep to the other to thereby control the architectureof the membrane.

In embodiments wherein the biointerface membrane is directly writtenonto the sensor, the dispenser is programmed to write a pattern thatcreates the desired membrane architecture, including the interconnectedcavities and solid portion(s). Namely, the dispenser is programmed tomove in the x, y, and optionally z direction in order to create thedesired membrane architecture. See, for example, U.S. Publication No.2004/0253365 A1 cited above.

Although the preferred embodiments described moving the dispenser(s)relative to the sensor, alternatively, the dispenser can remainstationary and the sensor moved, as is appreciated by one skilled in theart.

In some embodiments, the sensor is moved in a rotational ortranslational motion, which can be performed in combination with, orinstead of, movement of the dispenser. In this step, the sensor is movedso as to ensure coating throughout the entirety of the biointerfaceregion (or a portion thereof). In one exemplary embodiment, wherein asubstantially circumscribing biointerface membrane is desired (e.g., fora substantially cylindrically shaped sensing sensor) such as illustratedin FIG. 2A, the sensor can be rotated so to aid in coating the entirecircumference of the sensor. In another exemplary embodiment, wherein asubstantially planar biointerface membrane is desired (e.g., for asubstantially planar sensor), the sensor can be translated so as to aidin coating the desired planar surface area.

FIG. 6 is a flow chart that illustrates the process 160 of forming abiointerface-coated sensor in an alternative embodiment. In thisembodiment, the interconnected cavities and solid portion(s) of thebiointerface membrane are amorphous in configuration, such asillustrated in FIGS. 4A and 4B, for example.

At block 162, a selectively removable porogen (e.g., porous mold) isformed by spraying, coating, rolling, or otherwise forming selectivelyremovable particles, for example, sugar crystals, onto the surface ofthe sensor. Additional examples of materials suitable as selectivelyremovable mold material include thermoplastic polymers such as waxes,paraffin, polyethylene, nylon, polycarbonate, or polystyrene innaturally available particles or processed into specific sizes, shapes,molded forms, spheres or fibers, salt or other particles which cannot bemade to inherently stick together coated with sugar, and certain drugcrystals such as gentamycin, tetracycline, or cephalosporins. Ingeneral, any dissolvable, burnable, meltable, or otherwise removableparticle which can be made to stick together could be used. Preferably,the particles have shapes and sizes substantially corresponding to thedesired cavity dimensions, such as described in more detail above. Insome embodiments, the particles are made to adhere to the sensor byenvironmental conditions, for example, humidity can be used to causesugar to adhere to the sensor.

In some embodiments, the particles are made to coalesce to provide thedesired interconnectivity between the cavities. In an exemplary poroussilicone embodiment, sugar crystals are exposed to a humid environmentsufficient to cause coalescence of the sugar crystals. In somealternative embodiments, other molds may be used in the place of theparticles described above, for example, coral, self-assembly beads,etched and broken silicon pieces, glass frit pieces, and the like.

At block 164, a material (e.g., a moldable or conformable material) isfilled or coated into the interconnected cavities of the mold usingmethods common in the art of polymer processing, for example, injecting,pressing, vacuuming, vapor depositing, extruding, pouring, and the like.Examples of materials suitable for the resulting porous device includepolymers, metals, metal alloys, ceramics, biological derivatives, andcombinations thereof, in solid or fiber form. In an exemplary poroussilicone embodiment, silicone is pressed into the interconnectedcavities of the mold.

At block 166, the material is substantially cured or solidified to formthe solid portion(s) of the biointerface membrane. Solidification of thematerial can be accelerated by supplying dry air (which may be heated)to the material, for example. Additionally, freezing, freeze drying orvacuum desiccation, with or without added heat, may also be utilized tocause the material to solidify. In some circumstances, a skin or anyexcess material can be removed (e.g., shaved, etched, or the like) aftercuring. In the exemplary porous silicone embodiment, an outer skin ofsilicone is removed to expose the interconnected cavities at an outersurface.

At block 168, the selectively removable porogen (e.g., porous mold) isdissolved, melted, etched, or otherwise removed, leaving interconnectingcavities within the solid portion. Preferably, the selectively removableporogen is readily removable without significantly altering the finalproduct (or product material). This removal may be by dissolution bysome solvent that does not significantly dissolve the final productmaterial. Alternatively, the mold material may be melted (or burned) outof the final product material if the melting point (or burning point) ofthe mold material is below that of the final product material. In theexemplary porous silicone embodiment, water is used to dissolve thesugar crystals.

FIG. 7 is a flow chart that illustrates the process 170 of forming abiointerface-coated small structured sensor in another alternativeembodiment. In this embodiment, the interconnected cavities and solidportion(s) of the biointerface membrane are amorphous in configuration,such as illustrated in FIGS. 4A and 4B, for example, and the solidportion is molded around the sensor.

At block 172, a selectively removable porogen is formed by filling ashaped cavity with selectively removable particles, for example, sugarcrystals, wherein the sensor is located within the shaped cavity, andwherein the selectively removable particles substantially surround thesensor. Additional examples of materials suitable as selectivelyremovable mold material are described with reference to block 162,above. In some embodiments, the shaped cavity mold is formed from aselectively removable material (e.g., sacrificial cavity mold) similarthe selectively removable particles described above. One such exampleincludes a tube formed from a dissolvable polymer. Alternatively, theshaped cavity can be a non-selectively removable material, and instead,a sacrificial layer of selectively removable material is formed directlyonto the cavity walls, enabling the removal of the biointerface membraneafter dissolution of the sacrificial layer.

Preferably the shape of the cavity mold substantially corresponds to thedesired final shape of the biointerface membrane. In one exemplaryembodiment, the cavity mold is substantially cylindrical, for exampleusing a syringe or cannula as the cavity mold.

In some embodiments, the particles are made to coalesce to provide thedesired interconnectivity between the cavities. In an exemplary poroussilicone embodiment, sugar crystals are exposed to humidity or spray ofwater sufficient to cause coalescence of the sugar crystals. In somealternative embodiments, other molds may be used in the place of theparticles described above, for example, coral, self-assembly beads,etched and broken silicon pieces, glass frit pieces, and the like.

At block 174, a material (e.g., a moldable or conformable material) isfilled into the interconnected cavities of the mold using methods commonin the art of polymer processing, for example, injecting, pressing,vacuuming, vapor depositing, pouring, and the like. Examples ofmaterials suitable for the resulting porous device are described in moredetail with reference to block 164, above. In an exemplary poroussilicone embodiment, silicone is pressed into the interconnectedcavities of the mold.

At block 176, the material is substantially cured or solidified to formthe solid portion(s) of the biointerface membrane. Solidification of thematerial can be accelerated as described in more detail with referenceto block 166, above.

At block 178, the selectively removable porogen is dissolved, melted,etched, or otherwise removed, leaving interconnecting cavities withinthe solid portion surrounding the sensor. In some embodiments, wherein asacrificial layer is formed as described above, the sacrificial layercan be removed before, during, or after the removal of the selectivelyremovable porogen. In some embodiments, the final product is removedfrom the cavity mold before, during, or after the removal of theselectively removable porogen.

Preferably, the selectively removable porogen is readily removablewithout significantly altering the final product (or product material).This removal may be by dissolution by some solvent that does notsignificantly dissolve the final product material. Alternatively, themold material may be melted (or burned) out of the final productmaterial if the melting point (or burning point) of the mold material isbelow that of the final product material. In one exemplary embodiment, asacrificial tube forms the mold cavity; wherein the sacrificial tube isremoved prior to, during, or after dissolution of the selectivelyremovable porogen. One skilled in the art can appreciate a variety ofmodifications or combinations of the above described removal stepwithout departing from the spirit of the invention.

FIG. 8 is a flow chart that illustrates the process 180 of forming abiointerface-wrapped sensor in one embodiment. In this embodiment, theinterconnected cavities and solid portion(s) of the biointerfacemembrane can be fibrous or amorphous in configuration. In fact,substantially any biointerface membrane with an architecture asdescribed in more detail above, which is formed in substantially anymanner, can be used with this embodiment.

At block 182, a sensor is manufactured and provided, wherein the sensoris formed with a small structure as defined herein.

At block 184, a biointerface membrane with an architecture as describedherein is manufactured in substantially any desired manner, wherein thebiointerface membrane is formed substantially as a sheet or tube ofmembrane. Biointerface membranes suitable for wrapping around the sensorand providing the desired host interface are described in more detailabove (see section entitled, “Architecture of the First Domain.”)

At block 186, the biointerface membrane is wrapped around the sensormanually, or using an automated device, as can be appreciated by oneskilled in the art. Namely, the biointerface membrane is wrapped suchthat it substantially surrounds the sensor, or the sensing mechanism ofthe sensor (e.g., the electroactive surfaces or sensing membrane). Thenumber of wraps can be from less than 1 to about 100, preferably 1, 1½,2, 2½, 3, 3½, 4, 5, 6, 7, 8, 9, 10, or more. The number of wraps dependson the architecture of the sheet of biointerface membrane, and thedesired architecture of the biointerface surrounding the sensor.

In some embodiments, the circumference (or a portion thereof (e.g., anedge)) of the biointerface membrane with an architecture as describedherein can be adhered or otherwise attached or sealed to form asubstantially consistent outer surface (of the biointerface membrane).In an aspect of this embodiment, the biointerface membrane is wrappedaround the sensor one time, wherein the “wrap” includes a tubularbiointerface membrane configured to slide over the sensor (or sensingmechanism), for example, be stretching the tubular biointerface membraneand inserting the sensor therein.

FIG. 9 is a flow chart that illustrates the process 190 of forming asensing biointerface in one embodiment. In this embodiment, the sensoris inserted into the biointerface membrane so that it is encompassedtherein.

At block 192, a biointerface membrane is manufactured in substantiallyany desired manner. Biointerface membranes suitable for the sensingbiointerface are described in more detail above (see for example,section entitled, “Architecture of the First Domain”). In someembodiments, the biointerface membrane is molded into the desired finalshape to surround the sensor and implant into a host. Alternatively, thebiointerface membrane can be provided as a sheet of bulk material.

At block 194, a particularly shaped or sized biointerface membrane canbe (optionally) cut. Namely, in embodiments wherein the biointerfacemembrane is provided in bulk, e.g., as a sheet of material, the desireshape or size can be cut there from. In these embodiments, bulkbiointerface membrane sheet is preferably of the appropriate thicknessfor the desired final product. In one exemplary embodiment, thebiointerface membrane (bulk sheet) is compressed, for example betweentwo substantially rigid structures, and the final size/shapebiointerface membrane cut there from, after which the biointerfacemembrane is released. While not wishing to be bound by theory, it isbelieved that by compressing the biointerface membrane during cutting, amore precise shape can be achieved. It is noted that biointerfacemembranes can have sufficient elasticity, such that the thickness isreturned after release from compression, as is appreciated by oneskilled in the art.

At block 196, a sensor is inserted into the biointerface membrane.Preferably, the sensor is inserted into the membrane such that thesensing mechanism contacts at least one or more of the interconnectedcavities so that the host analyte can be measured. Alternatively, thebiointerface can be formed from a material that allows the flux of theanalyte there through. In some embodiments, the sensor is inserted withthe aid of a needle. Alternatively, the sensor is formed withappropriate sharpness and rigidity to enable insertion through thebiointerface membrane.

In some embodiments, an anchoring mechanism, such as a barb, is providedon the sensor, in order to anchor the sensor within the biointerfacemembrane (and/or host tissue). A variety of additional or alternativeaspects can be provided to implement the biointerface membranesurrounded sensors of the preferred embodiments.

A porous membrane material applied to the sensor can act as a spacerbetween the sensor and the surrounding tissue at the site of sensorinsertion, in either the short-term or long-term sensors. For example, aspacer from 60-300 microns thick can be created of porous siliconehaving pore sizes of 0.6 microns and greater (e.g., up to about 1,000microns or more). When inserted into the tissues, the adipose cells cometo rest against the outermost aspects of the porous membrane, ratherthan against the surface of the sensor (FIG. 1C), allowing open spacefor transport of water-soluble molecules such as oxygen and glucose.

Porous membrane material can be manufactured and applied to a sensorusing any advantageous method known to one skilled in the art. Asdiscussed elsewhere, porous membranes can be manufactured from a varietyof useful materials known in the art, depending upon the desiredmembrane parameters.

FIG. 10A is a scanning electron micrograph showing a cross-section of anexemplary porous silicone tube that does not contain a sensor. Note theopen porous structure of cavities and channels within the solidifiedsilicone. Porous silicone can be manufactured and applied to the sensorby a variety of means. The material in FIG. 10A, for example, was formedby sieving sugar to give crystals having a size and shape approximate tothat of the desired pore size. The sugar was humidified and thencompressed into a mold. The mold was then baked, to harden the sugarwithin the mold. Silicone was forced into the mold and then cured. Afterthe silicone was cured, the mold was removed and the sugar dissolvedaway. A sensor could subsequently be inserted into the porous siliconetube.

FIG. 10B is a scanning electron micrograph of sugar molded onto asensor. In this example, a sugar mold was formed directly on the sensor.Note the clumps of sugar crystals attached to the surface of the sensor.In this example, the sensor was placed into the mold, which was thenfilled with humidified sugar crystals. The mold containing the sensorand sugar was baked to solidify the sugar on the surface of the sensor.The sensor, with sugar crystals attached, was removed from the mold, inorder to prepare the electron micrograph. In some embodiments, thesensor can be rolled in the humidified sugar, to attach a layer of sugarto the sensor surface, and then baked to solidify the sugar. In someembodiments, the sugarcoated sensor may be rolled in humidified sugaradditional times to form a thicker sugar mold (e.g., two or more layers)around the sensor. In some embodiments, silicone is pumped or injectedinto the solidified sugar and cured. After curing, the sugar is removed,such as by washing, to give a porous silicone covered sensor.

In an alternative embodiment, porous silicone is pre-formed as a sheetor plug and then applied to the sensor. For example, a sugar moldlacking a sensor therein is formed using the usual means. As previouslydescribed, silicone is injected into the mold and then cured. After themold material is removed from the cured silicone, the sensor is insertedinto the plug, thereby creating a sensor having a porous siliconebiointerface membrane.

Alternatively, a thin sheet of porous silicone is manufactured and thenwrapped around the sensor. For example, a thin porous silicone sheet ismanufactured by pressing a thin layer of sieved, humidified sugar into aPetri dish. The sugar is baked. Silicone is applied to the sugar mold byinjection, pressing, or the like, and then cured. The sugar is removedfrom the porous silicone sheet, such as by washing. The manufacturedporous silicone is then wrapped around the sensor to form a biointerfacemembrane of a desired thickness.

In still another embodiment, other materials can be used to manufacturethe biointerface membrane. For example, the sensor can be wrapped in alayer of ePTFE having a pore size of about 0.6 microns and above, tocreate a layer about 12-100 microns thick. See U.S. patent applicationSer. No. 09/916,858 to Shults et al., entitled “DEVICE AND METHOD FORDETERMINING ANALYTE LEVELS.” In yet another embodiment, the spacer canbe either a smooth or porous hydrogel.

All references cited herein, including but not limited to published andunpublished applications, patents, and literature references, areincorporated herein by reference in their entirety and are hereby made apart of this specification. To the extent publications and patents orpatent applications incorporated by reference contradict the disclosurecontained in the specification, the specification is intended tosupersede and/or take precedence over any such contradictory material.

The term “comprising” as used herein is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps.

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification are to be understood as beingmodified in all instances by the term “about.” Accordingly, unlessindicated to the contrary, the numerical parameters set forth herein areapproximations that may vary depending upon the desired propertiessought to be obtained. At the very least, and not as an attempt to limitthe application of the doctrine of equivalents to the scope of anyclaims in any application claiming priority to the present application,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding approaches.

The above description discloses several methods and materials of thepresent invention. This invention is susceptible to modifications in themethods and materials, as well as alterations in the fabrication methodsand equipment. Such modifications will become apparent to those skilledin the art from a consideration of this disclosure or practice of theinvention disclosed herein. Consequently, it is not intended that thisinvention be limited to the specific embodiments disclosed herein, butthat it cover all modifications and alternatives coming within the truescope and spirit of the invention.

What is claimed is:
 1. An analyte sensing device adapted for short terminsertion into a host's soft tissue, comprising: a sensor having anarchitecture with at least one dimension less than about 1 mm, whereinthe architecture is configured to create a fluid-filled pocketsurrounding at least a portion of the sensor in vivo.
 2. The device ofclaim 1, wherein the sensor is configured to measure a signal that isindicative of a concentration of the analyte within the fluid-filledpocket.
 3. The device of claim 1, further comprising electronicsoperatively coupled to the sensor and configured for detecting a signalfrom the sensor, wherein the signal is indicative of a concentration ofan analyte within the host.
 4. The device of claim 3, further comprisinga housing configured for placement adjacent to a host's skin, wherein atleast a portion of the electronics are disposed in the housing.
 5. Thedevice of claim 1, wherein the sensor is a transcutaneous sensor.
 6. Thedevice of claim 1, further comprising a spacer covering at least aportion of the sensor.
 7. The device of claim 6, wherein the spacercovers a sensing mechanism of the sensor.
 8. The device of claim 6,wherein the spacer comprises a biointerface.
 9. The device of claim 6,wherein the spacer comprises a porous membrane configured to allow abody fluid to fill pores to thereby form a fluid-filled pocket.
 10. Thedevice of claim 1, further comprising a bioactive agent incorporatedinto the sensor.
 11. The device of claim 10, wherein the bioactive agentis selected from the group consisting of an anti-barrier cell agent, ananti-inflammatory agent, an anti-infective agent, a necrosing agent, ananesthetic, an inflammatory agent, a growth factor, an angiogenicfactor, an adjuvant, an immunosuppressive agent, an antiplatelet agent,an anticoagulant, an ACE inhibitor, a cytotoxic agent, a vascularizationcompound, and an anti-sense molecule.
 12. An analyte sensor formeasuring an analyte in a host, the sensor comprising: a sensorconfigured for transcutaneous insertion into a host's skin, wherein thesensor has an architecture with at least one dimension less than about 1mm; and a biointerface covering at least a portion of the sensor. 13.The device of claim 12, wherein the biointerface is configured to allowfluid influx.
 14. The device of claim 12, wherein the sensor is atranscutaneous sensor.
 15. The device of claim 12, further comprisingelectronics operatively coupled to the sensor and configured fordetecting a signal from the sensor.
 16. The device of claim 15, whereinthe electronics are inductively coupled to the sensor.
 17. The device ofclaim 12, further comprising a housing configured for placement adjacentto the host's skin.
 18. The device of claim 12, further comprising abioactive agent incorporated into the sensor.
 19. The device of claim18, wherein the bioactive agent is selected from the group consisting ofan anti-barrier cell agent, an anti-inflammatory agent, ananti-infective agent, a necrosing agent, an anesthetic, an inflammatoryagent, a growth factor, an angiogenic factor, an adjuvant, animmunosuppressive agent, an antiplatelet agent, an anticoagulant, an ACEinhibitor, a cytotoxic agent, a vascularization compound, and ananti-sense molecule.
 20. The device of claim 12, wherein thebiointerface is configured to provide a space for a body fluid to residearound the sensor in vivo.